[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/+/84956


Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>

typo3/sysext/form/Documentation/I/Concepts/Variants/Index.rst

@@ -210,6 +210,29 @@ For example:
 :yaml:`finisherIdentifier == "EmailToSender"`.
 
 
+``site`` (object)
+^^^^^^^^^^^^^^^^^
+
+You can access every public method from :php:`\TYPO3\CMS\Core\Site\Entity\Site`
+to access the following important ones:
+
+* getSettings() / The site settings array
+* getDefaultLanguage() / The default language object for the current site
+* getConfiguration() / The whole configuration of the current site
+* getIdentifier() / The identifier for the current site
+* getBase() / The base URL for the current site
+* getRootPageId() / The ID of the root page of the current site
+* getLanguages() / An array of available language for the current site
+* getSets() / Configured sets of a site (new in TYPO3 v13+)
+
+For example:
+
+:yaml:`site("settings").get("myVariable") == "something"`.
+:yaml:`site("rootPageId") == "42"`.
+
+More details on the `Site` object can be found in
+:ref:`Using site configuration in conditions <t3coreapi:sitehandling-inConditions>`.
+
 ``siteLanguage`` (object)
 ^^^^^^^^^^^^^^^^^^^^^^^^^