feature/build only branches with legacy asciidoc

[reakaleek]

Changes

Only build 8.x target branches.

Context

According to https://github.com/elastic/buildkite-pr-bot?tab=readme-ov-file#configuration-1 you can actually set the target_branch in pull-requests.org-wide.json.

However, the buildkite/docs-build-pr status check is usually a required check.

In this PR we exit early if the target branch doesn't match the pattern, so that the status check will still be green.

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[shainaraskas]

We have other docs sets that are not on stack versioning that wouldn't be caught in this fix. This would help a lot with certain repos but I wonder if it needs to be refactored to cover stuff like ECE 4+ and ECK 3+

[bmorelli25]

I'm trying to think through Shaina's comment. We could potentially pair Jan's change here with the repo-specific instructions that start on L68. We could write a script to parse conf.yaml and spit out a mapping of each repo and the versions of documentation that are contained within that repo. We could then check against those versions.

For example:

    # repositories with a docs dir and changelog
    "apm-aws-lambda" | "apm-agent-android" | "apm-agent-nodejs" | "apm-agent-python" | "apm-agent-ruby" | "apm-agent-rum-js" | "apm-agent-go" | "apm-agent-java" | "apm-agent-dotnet" | "apm-agent-php" | "apm-agent-ios")
+      if ! [[ $GITHUB_PR_TARGET_BRANCH =~ {NEWREGEXHERE} ]]; then
+       echo "Target branch '$GITHUB_PR_TARGET_BRANCH' is not a valid version branch (must be in format
+     X.Y where X <= 8)"
+      exit 0
+        fi
      git fetch origin "$GITHUB_PR_TARGET_BRANCH"
      docs_diff=$(git diff --stat "origin/$GITHUB_PR_TARGET_BRANCH"...HEAD -- "./docs/**.asciidoc" CHANGELOG.asciidoc)
      ;;

(this is just to illustrate)