Site admin area: Docs link should be more prominent & should link to product version

Created by: slimsag

It would be better if the docs link on /site-admin was more prominent. I challenge you to find it :) Something like Help for this version would be better.

Also, it should link to the Sourcegraph product version e.g. https://docs.sourcegraph.com/@3.3 depending on the version you are running so as to ensure users running older versions like 2.x today are able to easily find relevant docs.

Reported by https://app.hubspot.com/contacts/2762526/company/419771425/