[DOCS] Add details about how to extend the admin panel

This commits adds the following hints:
* module classes needs to be set to public
* module classes needs to implement the ShortInfoProviderInterface
* description on how to add custom page settings
* description on how to add custom module settings

Resolves: #104242
Releases: main, 12.4
Change-Id: Id495359e007bf09e38a80bc1097c95946f964217
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84962


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

typo3/sysext/adminpanel/Documentation/Extending/Index.rst

@@ -26,7 +26,7 @@ To create your own Admin Panel module
 =====================================
 
 #. Create a new PHP class extending
-   `\TYPO3\CMS\Adminpanel\ModuleApi\AbstractModule`.
+   `\TYPO3\CMS\Adminpanel\ModuleApi\AbstractModule` and implementing `TYPO3\CMS\Adminpanel\ModuleApi\ShortInfoProviderInterface`. If you don't implement the `ShortInfoProviderInterface`, your module will not be shown in the module menu.
 
 #. Implement at least the following methods:
 
@@ -39,7 +39,8 @@ To create your own Admin Panel module
    *  `getShortInfo` - Displayed next to the module label, may contain
       aggregated infos (such as `Total Parse Time: 200ms`)
 
-#. Register your module by adding the following in your `ext_localconf.php`
+#. Register your module by adding the following in your `ext_localconf.php`. Using `before` or `after` you can influence where your module will be
+displayed in the module menu by referencing the identifier / array key of other modules.
 
    .. code-block:: php
 
@@ -48,9 +49,7 @@ To create your own Admin Panel module
           'before' => ['cache'],
       ];
 
-Using `before` or `after` you can influence where your module will be
-displayed in the module menu by referencing the identifier / array key of
-other modules.
+#. Ensure that your module class is defined as 'public' in the `Services.yaml` file.
 
 Modules themselves do provide settings for the page rendering and global
 actions (like preview settings, clearing caches or adding action buttons for
@@ -59,9 +58,39 @@ editing the page) but do not provide further content.
 If you want to display additional content in the Admin Panel (like rendering
 times or backtraces), you have to add a submodule to your main module.
 
-To provide settings in a module, implement the method `getSettings`, which has
-to return rendered HTML form elements (but without the form tag), ready to be
-used in the Admin Panel main form.
+Adding settings
+===============
+
+Page settings
+-------------
+
+Page settings, like "Preview" or "Cache", are accessible via the "Settings" section in the module menu.
+
+To provide custom page settings, your main module needs to implement the `\TYPO3\CMS\Adminpanel\ModuleApi\PageSettingsProviderInterface` interface.
+The corresponding method `getPageSettings()` returns rendered HTML form elements (but without the form tag).
+
+Examples
+~~~~~~~~
+
+* `\TYPO3\CMS\Adminpanel\Modules\CacheModule`
+* `\TYPO3\CMS\Adminpanel\Modules\PreviewModule`
+
+
+Module settings
+---------------
+
+Module settings are accessible via the cogwheel-button in the opened module.
+
+To provide settings, your submodule needs to implement the `\TYPO3\CMS\Adminpanel\ModuleApi\ModuleSettingsProviderInterface` interface.
+The respective method `getSettings()` returns rendered HTML form elements (but without the form tag).
+
+Examples
+~~~~~~~~
+
+* `\TYPO3\CMS\Adminpanel\Modules\Debug\Log`
+* `\TYPO3\CMS\Adminpanel\Modules\TsDebug\TypoScriptWaterfall`
+
+
 
 Adding a sub-module
 ===================