[EuiMarkdownEditor] Enhancement request: Smarter UI when no plugins present

[i-a-n]

I notice after #4383 was merged, the markdown editor having zero UI plugins became a possibility; but the UI doesn't really handle that state gracefully:

The language and the horizontal rules are now somewhat misleading and out of place. If we updated the UI logic to be a bit smarter I think it'd make for a nicer experience.

[elizabetdev]

@chandlerprall we should also document better how to unregister plugins. It seems that @i-a-n found how to do it by reading this PR #4383:

https://github.com/elastic/dream-machine/blob/390d52ce81dcd1ac45727f64b992bd89763a1bb8/client/frontend/src/containers/CaseContainer/index.tsx#L610-L613