Add Backport automation for separately versioned content

[bmorelli25]

This is a WIP. Feedback is very welcome.

Summary

This PR adds two new automations to the observability-docs repo:

• add-backport-labels.yml
• clean-backports.yml

Contributor flow

This PR proposes the following contributor flow:

1. A contributor opens a PR that adds content to this repository. When opening the PR, the contributor will add one or more deployment-type labels: serverless, stateful, or cloud. Note that cloud is not currently relevant to this repo.
2. After the PR is reviewed and merged, the add-backport-labels.yml automation will run. This automation uses the label-mapping.yml file to map the deployment-type labels to the relevant backport branches. For example, if the PR is labeled with stateful, the stateful label would be removed and replaced with the backport-8.13 and backport-8.12 labels.
3. Mergify automatically opens PRs to the correct branches based on the newly added backport-* labels.
4. On these backport PRs, a new automation, clean-backports.yml will run. This automation uses regex to match the base branch of the backport PR with any block-listed directories. In this PR, backport-mapping.yml block-lists the serverless dir for branches that match the following regex pattern: [7,8].*. This means if our original PR had both serverless and stateful content, and has a backport that is targeting 8.13, an automation will run that rm -rf serverless on the backport PR. This ensures serverless content is not backported to branches other than main even when it exists in a stateful PR.
5. The backport PR passes checks and can be merged.

Problems solved

• Contributors don't need to know relevant backport branches. All they need to know is if the content applies to serverless, stateful, cloud, or apm-agents.
• Contributors don't have to open separate PRs for separately-versioned content. Any content can go into any PR and will make it to the appropriate backport branches.

[mergify]

This pull request does not have a backport label. Could you fix it @bmorelli25? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[bmorelli25]

Replaced (for now) with #3916