Create Kibana GitHub page for temporarily publishing UI Framework documentation

[cjcenizal]

We need to surface UI Framework documentation in an easily-accessible location, so that engineers can use the framework effectively.

In the long-term, we want to publish these docs on elastic.co (#9457), but that will take a lot of work. and our UI Framework isn't in a state where we're prepared to officially support it for plugin developers, so maybe it doesn't make sense to publish it in a prominent "official" place like elastic.co right now.

A GitHub page seems like a great short-term solution because it's in an obvious location, it's tied to the Kibana repo, and it's easy to deploy.

Questions:

1. Does this make sense / is it a good idea? Are there alternatives?
2. If this is a suitable approach, then is it appropriate for UI Framework-specific docs to live at elastic.github.io/kibana? (See elastic.github.io/kibana-ui-framework to see what the content would look like).
3. If not, then is it sufficient to just put the UI Framework docs behind a general Kibana landing page at that URL?
4. Would we ever publish other documentation here which is useful to us internally and which we want to share publicly (for the sake of transparency), but we don't want to give it the official stamp of approval by putting it on elastic.co, e.g. plugin API docs?

[cjcenizal]

CC @uboness

[cjcenizal]

Closing this in favor of #9457