Add first draft of "Available commands" page

docs/writing/available-commands.md

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+# Available commands
+
+The fixture plugin provides a few [symfony commands](#todo) you can execute.
+
+## Running fixtures
+### Run all fixtures
+```shell:no-line-numbers
+bin/console fixture:load
+```
+
+Possible arguments:
+| Argument | Description                                                | Default Value |
+| -------- | ---------------------------------------------------------- | ------------- |
+| --dry    | Display all fixtures that would run without executing them | false         |
+| --vendor | Execute also all fixtures found in the vendor directory    | false         |
+
+### Run a specific fixture
+```shell:no-line-numbers
+bin/console fixture:load:single {FixtureName}
+```
+
+Possible arguments:
+| Argument            | Description                                                | Default Value |
+| ------------------- | ---------------------------------------------------------- | ------------- |
+| --dry               | Display all fixtures that would run without executing them | false         |
+| --vendor            | Execute also all fixtures found in the vendor directory    | false         |
+| --with-dependencies | Also execute all dependencies recursively                  | false         |
+| -w                  | Alias for `--with-dependencies`                            | false         |
+
+### Run a fixture group
+```shell:no-line-numbers
+bin/console fixture:load:group {GroupName}
+```
+
+Possible arguments:
+| Argument | Description                                                | Default Value |
+| -------- | ---------------------------------------------------------- | ------------- |
+| --dry    | Display all fixtures that would run without executing them | false         |
+| --vendor | Execute also all fixtures found in the vendor directory    | false         |
+
+## Helper commands
+### Get random UUID
+```shell:no-line-numbers
+bin/console fixture:uuid
+```
+
+This command just returns a valid, random UUID.