'Help' utility

Both Vim and Emacs have extensive help functionality built into the editor; this has been a great asset for anyone learning the systems and for discoverability of features. [Kakoune](https://github.com/mawww/kakoune) has even added a 'clippy' like assistant which suggests things as you type!

These are all great things, Rasa has it's own challenges, given that it's a bit more fragmented than other ecosystems we'll need to find a way to allow extensions to extend the documentation, preferably in a way that's searchable or discoverable in some way.

This issue is for discussing possibilities or implementations.

The issue names no files, tests, or concrete entry point. Begin by reviewing Rasa's extension and documentation systems, then define how extensions can add searchable or discoverable help; completion criteria are not specified and need maintainer agreement.

Written by the indexing model from the issue text.