[9.18] fix: doc: Remove the CHANGES file

With the introduction of the generated changelog, the CHANGES file
became a symlink to doc/arm/changelog.rst. After the changes made in
!9549, the changelog file transitioned from being a wholly generated
file to one that includes versioned changelog files, which are
themselves generated. However, while implementing !9549, we overlooked
that the CHANGES file is copied to a release directory on an FTP server
and contains just "include" directives, not the changelog itself.
Therefore, in the same fashion as the "RELEASE-NOTES*.html" file, create
a "CHANGELOG*.html" file that redirects to the Changelog appendix of the
ARM.

Closes #5000

Backport of MR !9690

Merge branch 'backport-5000-provide-correct-changelog-on-ftp-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!9704