[TASK] EXT:form - Update and optimize documentation

The documentation of EXT:form is outdated. The whole documentation has
been proof read and revised. Furthermore, the document has experienced
a huge structural change. The long pages have been split into smaller
parts.

A lot of content was duplicated. Following the DRY principle, the
descriptions of functions/ attributes etc. are now documented at a
central place and references are used.

Resolves: #69346
Releases: master,7.6,6.2
Change-Id: I45ddcf2f93c94f2982ac5fdecd5a942fad2eef21
Reviewed-on: https://review.typo3.org/47060


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>