[TASK] Add documentation for AssetCollector

This adds phpdoc for the public AssetCollector::add...() functions and
notes for users migrating from PageRenderer::add...().

Resolves: #90870
Releases: master
Change-Id: I8896b83f172a0e70ff5bdae61106c3a66d29667c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63970


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>