Add ability to add intro message to MediaWiki:Rules page

Author: JeroenDeDauw

phpcs.xml

@@ -7,6 +7,7 @@
 
 	<rule ref="./vendor/mediawiki/mediawiki-codesniffer/MediaWiki">
 		<exclude name="Generic.Files.LineLength.TooLong" />
+		<exclude name="Generic.WhiteSpace.LanguageConstructSpacing.IncorrectSingle" />
 		<exclude name="MediaWiki.Commenting.FunctionComment" />
 		<exclude name="MediaWiki.Commenting.PropertyDocumentation" />
 		<exclude name="MediaWiki.Commenting.FunctionAnnotations.UnrecognizedAnnotation" />

src/EntryPoints/RulesHooks.php

@@ -46,7 +46,12 @@ public function onShowMissingArticle( $article ): void {
 	}
 
 	private function getRulesPageHtml(): string {
-		return Html::element( 'div', [ 'id' => 'ext-rules-app' ] ) .
+		return
+			Html::rawElement( 'p',
+				[ 'id' => 'ext-rules-intro' ],
+				wfMessage( 'ext-rules-intro' )->exists() ? wfMessage( 'ext-rules-intro' )->parse() : ''
+			) .
+			Html::element( 'div', [ 'id' => 'ext-rules-app' ] ) .
 			Html::rawElement( 'noscript', [], Html::noticeBox( wfMessage( 'rules-noscript-message' )->text(), '' ) );
 	}