[DOCS] Document `site` object usage in EXT:form symfony expressions

Currently only `siteLanguage` was documented as usable, now
also `site` is explained with the most common public getter methods
and an example.

Resolves: #99723
Releases: main, 12.4
Change-Id: I8cb6de70b481322c31bd6fa7b3319c74f8af517a
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84896


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>
Tested-by: Georg Ringer <georg.ringer@gmail.com>