Split the release notes into a separate file for each (active) major branch,

Split the release notes into a separate file for each (active) major branch,
as per my recent proposal.  release.sgml itself is now just a stub that should
change rarely; ideally, only once per major release to add a new include line.
Most editing work will occur in the release-N.N.sgml files.  To update a back
branch for a minor release, just copy the appropriate release-N.N.sgml
file(s) into the back branch.

This commit doesn't change the end-product documentation at all, only the
source layout.  However, it makes it easy to start omitting ancient information
from newer branches' documentation, should we ever decide to do that.