[BUGFIX] Fix site handling documentation

Since #86323 the structure of the site yaml file has been changed.
Fix the documentation accordingly.

Resolves: #86587
Releases: master
Change-Id: I776e06d9f790141507b0a884bf9e38f343b015a2
Reviewed-on: https://review.typo3.org/58589


Tested-by: TYPO3com <no-reply@typo3.com>
Tested-by: Christian Knauf <knauf@analog.de>
Reviewed-by: Josef Glatz <josef.glatz@typo3.org>
Tested-by: Josef Glatz <josef.glatz@typo3.org>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>