Automate docs release when external repos publish new docs

[ben-polinsky]

Building iTwin.js docs (itwinjs.org) requires source material from this repo (itwinjs-core) as well as external repos (appui, presentation, etc). When external repos publish new documentation, we need to manually kick off a docs build from core and trigger a release.

We can do a bit better by triggering a docs build in core (using the latest release branch) after any of these repos publishes new docs. This can, in turn, trigger a new release which will automatically deploy to dev.