docs: modify hermetic build docs (#3331)

In this PR:
- Add instructions on release note generation.
- Move generating libraries using docker image to `README.md`
- Move generating libraries using python scripts to `DEVELOPMENT.md`.
- Move instructions on building image to `DEVELOPMENT.md`.

hermetic_build/DEVELOPMENT.md

@@ -0,0 +1,228 @@
+> [!IMPORTANT]
+> All examples assume you are inside the repository root folder.
+
+
+# Linting
+
+When contributing, ensure your changes to python code have a valid format.
+
+```
+python -m pip install black
+black {source_file_or_directory}
+```
+
+# Install package dependencies
+
+```shell
+python -m pip install --require-hashes -r hermetic_build/common/requirements.txt
+python -m pip install hermetic_build/common
+python -m pip install --require-hashes -r hermetic_build/library_generation/requirements.txt
+python -m pip install hermetic_build/library_generation
+python -m pip install --require-hashes -r hermetic_build/release_note_generation/requirements.txt
+python -m pip install hermetic_build/release_note_generation
+```
+
+# Run the integration tests
+
+The integration tests build the docker image declared in
+`.cloudbuild/library_generation/library_generation.Dockerfile`, pull GAPIC
+repositories, generate the libraries and compare the results with the source
+code declared in a "golden branch" of the repo.
+
+It requires docker and python (>= 3.12.0) to be installed.
+
+```shell
+python -m unittest hermetic_build/library_generation/tests/integration_tests.py
+```
+
+# Run the unit tests
+
+There is one unit test file per component.
+Every unit test script ends with `unit_tests.py`.
+To avoid specifying them individually, we can use the following command:
+
+```shell
+python -m unittest discover -s hermetic_build -p "*unit_tests.py"
+```
+
+> [!NOTE]
+> The output of this command may look erratic during the first 30 seconds.
+> This is normal. After the tests are done, an "OK" message should be shown.
+
+# Run the library generation scripts in your local environment
+
+Although the scripts are designed to run in a Docker container, you can also
+run them directly.
+This section explains how to run the entrypoint script
+(`hermetic_build/library_generation/cli/entry_point.py`).
+
+## Assumptions made by the scripts
+
+### The Hermetic Build's well-known folder
+Located in `${HOME}/.library_generation`, this folder is assumed by the scripts
+to contain certain tools.
+
+Developers must make sure this folder is properly configured before running the
+scripts locally.
+Note that this relies on the `HOME` environment variable which is always defined
+as per [POSIX env var definition](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html).
+
+#### Put the gapic-generator-java jar in its well-known location
+
+1. Run the following command to install gapic-generator-java.
+
+   ```shell
+   mvn install -B -ntp -DskipTests -Dclirr.skip -Dcheckstyle.skip
+   ```
+   This will generate a jar located in `~/.m2/repository/com/google/api/gapic-generator-java/{version}/gapic-generator-java-{version}.jar`
+
+2. Move the jar into its well-known location.
+
+   ```shell
+   mv /path/to/jar "${HOME}/.library_generation/gapic-generator-java.jar"
+   ```
+
+#### Put the java formatter jar in its well-known location
+
+1. Download google-java-format-{version}-all-deps.jar from [Maven Central](https://central.sonatype.com/artifact/com.google.googlejavaformat/google-java-format)
+or [GitHub releases](https://github.com/google/google-java-format/releases).
+2. Move the jar into its well-known location.
+
+   ```shell
+   mv /path/to/jar "${HOME}/.library_generation/google-java-format.jar"
+   ```
+
+## Installing prerequisites
+
+In order to run the generation scripts directly, there are a few tools we
+need to install beforehand.
+
+### Install the owl-bot CLI
+
+This requires node.js to be installed.
+Check this [installation guide](https://github.com/nvm-sh/nvm?tab=readme-ov-file#install--update-script)
+for NVM, Node.js's version manager.
+
+After you install it, you can install the owl-bot CLI with the following
+commands:
+
+```shell
+git clone https://github.com/googleapis/repo-automation-bots
+cd repo-automation-bots/packages/owl-bot
+npm i && npm run compile && npm link
+owl-bot copy-code --version
+```
+
+The key step is `npm link`, which will make the command available in you current
+shell session.
+
+## Run the script
+The entrypoint script (`hermetic_build/library_generation/cli/entry_point.py`)
+allows you to generate a GAPIC repository with a given api definition (proto,
+service yaml).
+
+### Download the api definition
+For example, from googleapis
+
+```shell
+git clone https://github.com/googleapis/googleapis
+export api_definitions_path="$(pwd)/googleapis"
+```
+
+### Download the repo
+For example, google-cloud-java
+```shell
+git clone https://github.com/googleapis/google-cloud-java
+export path_to_repo="$(pwd)/google-cloud-java"
+```
+
+### Install the scripts
+
+You can skip this step if you've installed the packages in [Install package dependencies](#install-package-dependencies).
+
+```shell
+python -m pip install --require-hashes -r hermetic_build/common/requirements.txt
+python -m pip install hermetic_build/common
+python -m pip install --require-hashes -r hermetic_build/library_generation/requirements.txt
+python -m pip install hermetic_build/library_generation
+```
+
+### Run the script
+
+```shell
+python hermetic_build/library_generation/cli/entry_point.py generate \
+   --repository-path="${path_to_repo}" \
+   --api-definitions-path="${api_definitions_path}"
+```
+
+# Build the image from source
+
+1. Run the following command to build the image from source
+
+   ```shell
+   docker build \
+     -f .cloudbuild/library_generation/library_generation.Dockerfile \
+     -t local:image-tag \
+     .
+   ```
+
+2. Set the version of gapic-generator-java
+
+   ```shell
+   LOCAL_GENERATOR_VERSION=$(mvn \
+     org.apache.maven.plugins:maven-help-plugin:evaluate \
+     -Dexpression=project.version \
+     -pl gapic-generator-java \
+     -DforceStdout \
+     -q)
+   ```
+
+3. Run the image
+
+   ```shell
+      # Assume you want to generate the library in the current working directory
+      # and the generation configuration is in the same directory.
+      docker run \
+        --rm \
+        --quiet \
+        -u "$(id -u):$(id -g)" \
+        -v "$(pwd):/workspace" \
+        -v /path/to/api-definitions:/workspace/apis \
+        -e GENERATOR_VERSION="${LOCAL_GENERATOR_VERSION}" \
+        local:image-tag \
+        --generation-config-path=/workspace/generation_config_file \
+        --library-names=apigee-connect,asset \
+        --repository-path=/workspace \
+        --api-definitions-path=/workspace/apis
+      ```
+   Note that if you specify the generator version using environment variable,
+   `-e GENERATOR_VERSION="${LOCAL_GENERATOR_VERSION}"` in the above example,
+   you should not set `gapic_generator_version` and `protoc_version` in the
+   generation configuration because values in the generation configuration will
+   take precedence.
+
+# Debug the library generation container
+If you are working on changing the way the containers are created, you may want
+to inspect the containers to check the setup.
+It would be convenient in such case to have a text editor/viewer available. 
+You can achieve this by modifying the Dockerfile as follows:
+
+```dockerfile
+# install OS tools
+RUN apk update && apk add \
+	unzip curl rsync openjdk11 jq bash nodejs npm git less vim
+```
+
+We add `less` and `vim` as text tools for further inspection.
+
+You can also run a shell in a new container by running:
+
+```shell
+docker run \
+  --rm \
+  -it \
+  -u $(id -u):$(id -g) \
+  -v /path/to/google-cloud-java:/workspace \
+  --entrypoint="bash" \
+  $(cat image-id)
+```