[TASK] Add sitemap to sysext:core/Documentation

Resolves: #81477
Releases: master, 8.7, 7.6
Change-Id: Ie98fcc81c386fe5704c669f36220827f7cfbd2e5
Reviewed-on: https://review.typo3.org/53133


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>