[DOCS] Add README.rst as "about" file to system extensions

Fix links to system extension in composer.json documentation where it
was still missing. Remove reference to docs_indexed_search from indexed
search. It has not been updated since 2008.

Remove apostrophe from the verb "lets".

Resolves: #95816
Releases: master
Change-Id: I02a325f73c05c41d8cdf1270606cf20509151c71
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72014


Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Sybille Peters <sypets@gmx.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Sybille Peters <sypets@gmx.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>