Create lightweight usage docs for tabs-blocks

In https://antora.zulipchat.com/#narrow/stream/282400-users/topic/tabs-block.20extension Dan and I discussed how the tabs-block extension works.

I noted in this thread that there was not a lot of user assistance for this extension. Dan confirmed that the way this extension approaches the tabbed block solution is still considered the best way to define the blocks.

With this in mind I propose that the README is enhanced to include some of the usage information in this Zulip thread so that folks can better understand how to add this extension into the default-ui.