[DOCS] Create changelog structures for 12.0

New changelog entries now go to the release that they will be
integrated in, currently 12.0.
Updated the Howto.rst to adhere to current documentation standards
and use current branches and versions as examples. Updated the
section about where to put .rst files in the Changelog.

Fixes: #96036
Releases: master

Change-Id: Id5eef8caa3b53137507b283387884148ea8c63f2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72235


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>