Add documentation of the new RSS feed API

Closes #413

docs/migration/wsc60/php.md

@@ -28,3 +28,7 @@ class MyForm extends AbstractForm {
     }
 }
 ```
+
+## RSS Feeds
+
+A [new API](../../php/api/rss_feeds.md) for the output of content as an RSS feed has been introduced. 

docs/php/api/rss_feeds.md

@@ -0,0 +1,89 @@
+The RSS feed API makes it possible to output content in the RSS format in accordance with the [RSS 2.0 specifications](https://www.rssboard.org/rss-specification).
+The [ArticleRssFeedPage](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/page/ArticleRssFeedPage.class.php) is available as a reference implementation.
+
+## `RssFeed`
+
+[RssFeed](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/rssFeed/RssFeed.class.php) represents an RSS feed.
+
+After content has been added to the RSS feed, the feed can be output as XML using the `render` method.
+
+Example:
+
+```php
+$feed = new RssFeed();
+...
+$output = $feed->render();
+```
+
+## `RssFeedCategory`
+
+[RssFeedCategory](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/rssFeed/RssFeedCategory.class.php) represents a category of a feed item. A feed item can have multiple categories.
+
+An instance of `RssFeedCategory` is created implicitly when a category is assigned to a feed item.
+
+Example:
+
+```php
+$item = new RssFeedItem();
+$item->category('title', 'domain');
+```
+
+## `RssFeedChannel`
+
+[RssFeedChannel](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/rssFeed/RssFeedChannel.class.php) represents a channel within an RSS feed.
+
+Example:
+
+```php
+$feed = new RssFeed();
+$channel = new RssFeedChannel();
+$channel
+    ->title('title')
+    ->description('description')
+    ->link('https://www.example.net');
+$feed->channel($channel);
+```
+
+## `RssFeedEnclosure`
+
+[RssFeedEnclosure](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/rssFeed/RssFeedEnclosure.class.php) represents an enclosure of a feed item. A feed item can only have one enclosure.
+
+An instance of `RssFeedEnclosure` is created implicitly when an enclosure is assigned to a feed item.
+
+Example:
+
+```php
+$item = new RssFeedItem();
+$item->enclosure('url', /*size*/1024, 'mime/type');
+```
+
+## `RssFeedItem`
+
+[RssFeedItem](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/rssFeed/RssFeedItem.class.php) represents an item in an RSS feed.
+
+Example:
+
+```php
+$feed = new RssFeed();
+$channel = new RssFeedChannel();
+$feed->channel($channel);
+
+$item = new RssFeedItem();
+$item
+    ->title('title')
+    ->link('url');
+$channel->item($item);
+```
+
+## `RssFeedSource`
+
+[RssFeedSource](https://github.com/WoltLab/WCF/blob/master/wcfsetup/install/files/lib/system/rssFeed/RssFeedSource.class.php) represents the scource of a feed item. A feed item can only have one scource.
+
+An instance of `RssFeedSource` is created implicitly when an scource is assigned to a feed item.
+
+Example:
+
+```php
+$item = new RssFeedItem();
+$item->source('title', 'url');
+```

mkdocs.yml

@@ -40,6 +40,7 @@ nav:
           - 'Package Installation Plugins': 'php/api/package_installation_plugins.md'
           - 'User Activity Points': 'php/api/user_activity_points.md'
           - 'User Notifications': 'php/api/user_notifications.md'
+          - 'RSS Feeds': 'php/api/rss_feeds.md'
           - 'Sitemaps': 'php/api/sitemaps.md'
       - 'Code Style': 'php/code-style.md'
       - 'Apps': 'php/apps.md'