[DOCS] Remove "Adding Your Own Content Elements"

Content has been migrated to "TYPO3 Explained"

Reduce the content to a stub and add link.

Resolves: #89949
Releases: master, 9.5, 8.7
Change-Id: I4c69c8a1f74bea223110b9b6a07e035a03183d87
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/62641


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Björn Jacob <bjoern.jacob@tritum.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Björn Jacob <bjoern.jacob@tritum.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>