Assign better priorities to docs pages for searching, and include older docs

Assign better priorities to docs pages for searching, and include older docs

Instead of just including supported versions, include older versions of
the docs as well, and properly assign priorities by:

* For current docs, always assign priority 1.0
* For any other docs, start at 0.8 for the latest and decrease priority
  by 0.1 for each version, until we reach 0.1.

This should restore the ability to search for old versions of the
documentation (they still get proper priorities when searching for
suburls, which currently returns no hits at all) while maintaining
a strong priority for the newer versions.