feat: skip build for APM Server if there are no code changes

[kruskall]

docs/build-pr is the slowest job in apm-server CI pipeline and it is impacting significantly developer velocity when merging PRs. Update the buildkite pipeline to exit early if there are no docs changes.
APM Server docs is actually in a separate repository (observability-docs) and the few remaining files are barely touched (except for the changelog)

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[bmorelli25]

@kruskall — this is really neat!! We can potentially take this enhancement to many other repos to save both time and money. Do you or @v1v know how I can test this?

[kruskall]

@bmorelli25 you can extract the part of the script that checks the diff and run it locally.

To test it on apm-server:

I've checked a new branch, made a random commit with a code file. Result:

$ GITHUB_PR_BASE_REPO=apm-server GITHUB_PR_TARGET_BRANCH=main sh diff.sh 
main in apm-server has no docs changes

Update docs/README.md:

$ GITHUB_PR_BASE_REPO=apm-server GITHUB_PR_TARGET_BRANCH=main sh diff.sh 
diff:
 docs/README.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

If you agree with this approach I can add support for other repositories

[kruskall]

I'd suggest to add support for other repositories gradually