Handling previews for PRs against feature branches

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

For example, in https://github.com/elastic/elasticsearch/pull/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.