doc,tools: get altDocs versions from CHANGELOG.md

Parse `CHANGELOG.md` for versions of Node.js used by the documentation
feature `View another version` so that we don't have to manually update
the list when we cut a new version or transition a release to LTS.

PR-URL: https://github.com/nodejs/node/pull/27661


Reviewed-By: Rich Trott <rtrott@gmail.com>