Support for descriptions of tags

[djMax]

I would like descriptions on tag entities to be included in the documentation.

Context

Right now, the tags will create left bar groups for the methods, but you can't click on the tag itself and get the description of the tag.

Current Behavior

Tag description is lost.

Expected Behavior

Clicking on a tag should render the markdown for the tag description.

[kentbulza]

Upvote.

[Airkro]

Please help review the PR: https://github.com/stoplightio/elements/pull/2594 for this feature.