[DOCS] Document changed behavior with Composer and extensions

With #94996 the behavior for composer based installations changed:

- All extensions are always active
- PackageStates.php is not used anymore
- ext_emconf.php is not used anymore

More information makes updates easier for integrators.

Resolves: #95647
Releated: #94996
Releases: master
Change-Id: I24f727987484e9eb2cefec37d85293dc2657aa66
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71805


Tested-by: Simon Gilli <typo3@gilbertsoft.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Simon Gilli <typo3@gilbertsoft.org>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>