This mostly reverts commit 75b219d35a.
The changes to the image URLs in Alert.dox are still needed, so I
didn't revert those.
As per discussion on the mailing list and IRC.
* Remove references to outdated stuff (OsDrawer, old OpenGrok)
* Removed GitHub as a code search option (not nearly as flexible
as OpenGrok, and updated bi-monthly or so instead of daily.)
* General reorganization.
Fixes#11842.