#4: Add basic documentation about the tabs-block to the extensions repo.

If merged this MR adds basic documentation about how to use the tabs-block extension.

I have put very terse instructions that need some extra, more specific details about how to incorporate this extension into the default Antora UI project.

I'm hopeful that I managed to incorporate most of the key points in the Zulip chat relating to this, and as documented in #4 (closed)