Handling previews for PRs against feature branches

[lcawl]

It seems that documentation previews are not generated for feature branches.

For example, in elastic/elasticsearch#50821 the elasticsearch-ci/docs check is successful but doesn't generate previews and the log says:

15:21:37 INFO:build_docs:No changes to push

There are in fact documentation changes in that PR, however, so it seems to me that check is providing misinformation.

I think we should make it clearer somehow that folks need to do their own documentation builds to verify changes in these types of PRs. For example, maybe it's simplest to just disable these checks entirely on the branches that can't generate docs (so they're not giving false success indicators). Or else enable correct testing for all branches.

[nik9000]

It's be fairly easy to add the feature branch to the list of branches for which we build docs. Then the docs for the feature branch would be public and we'd build the docs on previews. The feels like the right kind of public for us.