2.6 changelog gen/version/root dir cleanup (ansible#40421)

* patched in changelog gen stuff from stable-2.5
* Makefile updates
* release.py as single-source-of-truth
* Remove obsolete ansible-core-sitemap.xml file.
* Move ROADMAP.rst into README.rst.
* dynamic rpm changelog, zap old deb/rpm changelogs
* fix changelog in MANIFEST.in
* Remove obsolete hacking/update.sh script.
* Remove ref to deleted authors script.
* Remove ref to removed module-formatter script.
* Update headings to match script names.
* MANIFEST.in cleanup
* removed RELEASES.txt and versions.yml
* removed obsolete release generation playbook/bits (not used since 2.5)
* misc Makefile cleanup
* speculative changes to DEB versioning
* allow override of DEB_VERSION/DEB_RELEASE

.gitattributes

@@ -1,4 +1,2 @@
-CHANGELOG.md merge=union
 .github/ export-ignore
 hacking/ export-ignore
-ticket-stubs/ export-ignore