doc: reorganize docsite homepage
Created by: bobheadxi
With the recent addition of the Cloud documentation
section (https://github.com/sourcegraph/sourcegraph/pull/24074), the nesting on the homepage is a bit incorrect, since features/tutorials are now under cloud documentation only:
This PR gets rid of items in the "core documentation" category in favour of more specific categories, so that the more general-use-case feature docs come first, then cloud, then self-hosted