Create automation to merge docs changes to the branches used for the site

[nventuro]

From #47

The docsite is deployed from specific branches of each repo (usually docs-v*). This is good because it prevents new documentation for unreleased features to show up on the docs.

However, when doing small fixes such as typos, it is desirable for that change to propagate to all deployed sites. We've been manually cherry-picking these commits into the doc branch, but this is cumbersome, and relies on maintainers remembering to do it.

It'd be great if we had a bot that automatically did the cherry-picking for PRs with a special label (docs-fix?). It'd perform this process silently, and only report back on the (merged) PR if it got a merge conflict when applying the cherry-pick.

[frangio]

A solid option is to implement this using GitHub Actions. github-script may come in handy, along with this article.