Conditionally run rebuild or incremental build

[bmorelli25]

Summary

In #2929, we reverted to incremental builds run every 30 minutes. This works great, until a PR is merged to the elastic/docs repository. Almost every PR merged in this repo requires a rebuild in order for the changes to take effect on elastic.co/guide/*.

This PR checks to see if the build commit of elastic/docs has changed since the previous scheduled run. If it has changed, we run a full docs rebuild. If it hasn't changed, there's no need for a full rebuild, and we instead run an incremental build.

I have no idea if this works or how to even begin testing it.

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[bmorelli25]

Replacing this with #2931 because I can't figure out how to test a fork and testing on upstream is easy.