Tidy up unlinked and obsolete files

A number of files in the `content/` directory are either unlinked (and
therefore hidden) or would have contained a message (generated by a wiki
template) stating they were obsolete and pointing to the updated location.

 - Move the unlinked documents into guidelines so that they are at least
   accessible via the automatic list, link if a quick location can be
   found.
 - Remove the obsolete pages and update the links to point to the correct
   location.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>