Docs: auto-build release notes for snapshots

Docs: auto-build release notes for snapshots

This adds conditional build support to generate release notes whenever
a tarball is being created, regardless of what the code branch status
is. All that is required is the linuxdoc tool chain.

Formal release branch snapshots have been publishing the notes files
built for their previous release. But development versions of Squid
have not been getting documented at all which can be annoying for
testers.

The release-N.html file is also removed from the repository. With this
update it should no longer be needed by the snapshot machinery.