This new pages accomplishes several interrelated things:
* Documents that Zulip Cloud runs master and how that works.
* Documents policies on how long client apps are expected to support
old releases in our compatibility matrix.
* Removes the 3-years-stale roadmap article.
* Provides a central place to talk about different versions in Zulip.
* Provides a better place to link to from our "you need to upgrade" nag.
This content is not intended to be final, but should be finalized in
the next week or so.
Fixes#18322.
Anders Kaseorg
Tim Abbott