[FEATURE] Document site sets (#4378)

* [FEATURE] Document site sets

Releases: main
Resolves: TYPO3-Documentation/Changelog-To-Doc#912, TYPO3-Documentation/Changelog-To-Doc#889
References: TYPO3-Documentation/Changelog-To-Doc#890,

* Apply suggestions from code review

Co-authored-by: Garvin Hicking <blog@garv.in>

* Update Documentation/ApiOverview/SiteHandling/SiteSets.rst

* Apply suggestions from code-review

* Apply suggestions from code-review

* Apply suggestions from code-review

---------

Co-authored-by: Garvin Hicking <blog@garv.in>

Documentation/ApiOverview/SiteHandling/Index.rst

@@ -1,6 +1,6 @@
-.. include:: /Includes.rst.txt
-.. index:: Site handling
-.. _sitehandling:
+..  include:: /Includes.rst.txt
+..  index:: Site handling
+..  _sitehandling:
 
 =============
 Site handling
@@ -10,21 +10,22 @@ The site handling defines entry points to the frontend sites of a
 TYPO3 instance, their languages and routing details. This chapter walks
 through the features of the module and goes into API and programming details.
 
-.. toctree::
-   :titlesonly:
+..  toctree::
+    :titlesonly:
 
-   Basics
-   CreateNew
-   BaseVariants
-   AddLanguages
-   ErrorHandling/Index
-   ErrorHandling/WriteCustomErrorHandler
-   StaticRoutes
-   UsingEnvVars
-   UseSiteInTypoScript
-   UseSiteInConditions
-   UseSiteInTCA
-   SiteSettings
-   CliTools
-   AccessingSiteConfiguration
-   ExtendingSiteConfig
+    Basics
+    CreateNew
+    BaseVariants
+    AddLanguages
+    ErrorHandling/Index
+    ErrorHandling/WriteCustomErrorHandler
+    StaticRoutes
+    UsingEnvVars
+    UseSiteInTypoScript
+    UseSiteInConditions
+    UseSiteInTCA
+    SiteSettings
+    SiteSets
+    CliTools
+    AccessingSiteConfiguration
+    ExtendingSiteConfig