Split out CSS-specific instructions from self-hosting overview
This fixes #689.
The move of infomration into more logical set of pages does not have to wait for test stuff to be decided, surely?
With the exception of updating as per https://github.com/solid/solidproject.org/pull/690#discussion_r864667235 , this PR is fine as far reorganising existing information goes. solidproject.org happens to include the instructions to help potential users but if that information is subject to going stale or better to link out to an authoritative document that's published by the CSS team somewhere and updated more frequently, then that can work too.