Document the site settings editor in all system
extension manuals that offer site sets.

* felogin
* fluid_styled_content
* indexed_search
* seo

While system extension EXT:form has a site set, that site set
offers no settings yet and therefore will not appear in the
settings editor.

Screenshots might be provided at a later time.

Resolves: #104926
Releases: main
Change-Id: Ide104b45bd7e7f58ff8e834f1520dd8442c908cb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/86347


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Praetorius <simon@praetorius.me>
Reviewed-by: Simon Praetorius <simon@praetorius.me>
Reviewed-by: Lina Wolf <112@linawolf.de>
Tested-by: Benjamin Franzke <ben@bnf.dev>