Figure out how to have an online version of the API Docs that match develop

[tig]

With the new branching model, the API Docs at https://gui-cs.github.io/Terminal.Gui/index.html will always represent main, which is correct.

However, when we are developing against develop we also want to be able view/test the API docs that match develop. Using a local docfx --serve version is not sufficient.

The ./README.md in develop should ALSO match develop and link to the develop version of the API Docs.

Related: Issue #1745

And see this: https://tehgm.net/blog/docfx-github-actions/#:~:text=I%20also%20recommend%20that%20you%20work%20on%20minimum,to%20only%20when%20a%20release%20is%20actually%20ready.

[tig]

This seems very hard to do. I'm not going to attempt it for 1.7.