[DOCS] Customization of the form editor

A new subchapter has been added to the form
documentation. It addresses a common
misconfiguration when removing form elements
from the form editor. In addition, the chapter
has been reorganized and a sphinx warning has
been fixed.

Resolves: #97103
Releases: main, 11.5
Change-Id: Ie90c6f4076d2b13675b9db686974efbb7f833bd6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73812


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>