[TASK] Change some site configuration details

Some first experiences with the new v9.2 site configuration
show us that people think site configuration can be extended
just like any other TCA, but that's not the case. The
patch changes some details to make more clear site
configuration is not just some other sort of TCA with the
same feature set.

* Do not prefix site_* with sys_ anymore to abstract
  better from db tables
* Rename configuration folder SiteConfigurationTca/
  to SiteConfiguration/ to make it more clear this
  is not straight TCA
* Do not load SiteConfiguration into GLOBALS['TCA']
  to distinct from TCA
* Mark extending site configuration experimental, add
  a documentation example and explain what is not working.

Change-Id: I87890ccd07ca73c4ef1f7690c61f8d2185baf575
Resolves: #85050
Releases: master
Reviewed-on: https://review.typo3.org/57001


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Mark...