[TASK] Document typo3-site-settings directive (#454)

Resolves TYPO3-Documentation/render-guides#716

Configuration/Sets/MySet/settings.definitions.yaml

@@ -0,0 +1,22 @@
+settings:
+  website.background.color:
+    label: 'Background color'
+    description: 'This will validate the given color string'
+    type: color
+    default: '#129845'
+
+  website.image.lazyLoading:
+    default: lazy
+    label: 'Browser-native image lazy loading'
+    type: string
+    enum:
+      lazy: 'Lazy'
+      eager: 'Eager'
+      auto: 'Auto'
+    description: 'Can be "lazy" (browsers could choose to load images later), "eager" (load images right away) or "auto" (browser will determine whether the image should be lazy loaded or not)'
+
+  website.rte.allowTags:
+    default: 'a, abbr, acronym, address, article, aside, b, bdo, big, blockquote, br, caption, center, cite, code, col, colgroup, dd, del, dfn, dl, div, dt, em, figure, font, footer, header, h1, h2, h3, h4, h5, h6, hr, i, img, ins, kbd, label, li, link, meta, nav, ol, p, pre, q, s, samp, sdfield, section, small, span, strike, strong, style, sub, sup, table, thead, tbody, tfoot, td, th, tr, title, tt, u, ul, var'
+    label: 'List of allowed HTML tags when rendering RTE content'
+    type: string
+    description: ''

Documentation/Reference/ReStructuredText/Code/SiteSettings.rst

@@ -0,0 +1,23 @@
+..  include:: /Includes.rst.txt
+..  _reference-site-settings:
+
+=============
+Site settings
+=============
+
+When an extension features :ref:`Settings definitions <t3coreapi:site-sets-settings-definition>`
+for site settings these can be automatically documented by including the settings
+definition file. The special syntax `PROJECT:` can be used to load the file
+from the directory of the extension instead of the `Documentation` folder only.
+
+..  literalinclude:: _siteSetSettings.rst.txt
+    :language: rst
+    :caption: Settings.rst
+
+..  include:: _siteSetSettings.rst.txt
+
+The custom parameters `:type:`, `:Label:` etc can be used to configure
+which aspects of the settings should be shown in the overview table.
+
+A `:name:` can be used to create a namespace when there are several sets. All
+links to the configuration values will be prefixed then.

Documentation/Reference/ReStructuredText/Code/_siteSetSettings.rst.txt

@@ -0,0 +1,5 @@
+..  typo3:site-set-settings:: PROJECT:/Configuration/Sets/MySet/settings.definitions.yaml
+    :name: my-set
+    :type:
+    :Label: max=30
+    :default: max=10

Documentation/guides.xml

@@ -13,7 +13,7 @@
                edit-on-github="TYPO3-Documentation/TYPO3CMS-Guide-HowToDocument"
                report-issue="https://github.com/TYPO3-Documentation/TYPO3CMS-Guide-HowToDocument/issues/new/choose"
                interlink-shortcode="h2document"
-               typo3-core-preferred="stable"
+               typo3-core-preferred="main"
     />
     <project title="Writing Documentation" release="main" version="main"
              copyright="since 2017 by the TYPO3 contributors"/>