Migrating Category wiki page

Moves an improved version of https://wiki.eclipse.org/Tycho/category.xml
to the Tycho documentation

See #2564

src/site/markdown/Category.md

@@ -0,0 +1,64 @@
+# Category
+
+A category.xml file can be used to define which content is placed into a p2 repository. 
+It can also specify how to display the content in the p2 installation dialog. 
+For Tycho to use it, it must to be placed into the root of an project with the packaging type 'eclipse-repository'.
+
+The 'category.xml' format was originally defined by the Eclipse PDE project. 
+There are extensions to the format only supported by p2 and Tycho.
+
+The following listing is a simple category file listing only one feature and one plug-in.
+
+
+```
+<?xml version="1.0" encoding="UTF-8"?>
+<site>
+   <feature id="com.example.feature">
+      <category name="com.example.category.update"/>
+   </feature>
+   <category-def name="com.example.category.update" label="Example update site"/>
+</site>
+```
+The following is an example, demonstrating a complex category definition.
+
+```
+<?xml version="1.0" encoding="UTF-8"?>
+<site>
+   <!-- Include features -->
+   <feature id="feature.id" version="1.4.100.v2009"/>
+   <!-- Since Tycho 1.1.0 (https://bugs.eclipse.org/bugs/show_bug.cgi?id=407273#c18), features can declare platform-specific compatibility for multi-platform builds -->
+   <!-- Examples: https://github.com/mickaelistria/org.eclipse.simrel.build/blob/master/categories/category.xml#L581 -->
+   <feature id="linux.specific.feature.id" version="0.0.0" os="linux"/>
+ 
+   <!-- Directly include bundles, without a feature -->
+   <bundle id="bundle.id" version="1.3.1.v2023"/>
+   <!-- Directly include any iu -->
+   <iu id="unit.id"/>
+   <!-- Include all IUs matching an expression -->
+   <iu>
+     <query> 
+       <expression type="match">
+         <![CDATA[
+           id == $0
+         ]]>
+       </expression>
+       <param>another.unit.id</param>
+     </query>
+   </iu>
+ 
+   <!-- Categories -->
+   <feature id="feature.in.category">
+      <category name="category.id"/>
+   </feature>
+   <category-def name="category.id" label="Category Label">
+      <description>Details on the category</description>
+   </category-def>
+ 
+   <!-- example for a dynamic category -->
+   <category-def name="javax" label="Bundles starting with javax."/>
+   <iu>
+      <category name="javax"/>
+      <query><expression type="match">id ~= /javax.*/</expression></query>
+   </iu>
+</site>
+```

src/site/site.xml

@@ -9,6 +9,7 @@
 			<item name="Target Platform" href="TargetPlatform.html" />
 			<item name="Testing Bundles" href="TestingBundles.html" />
 			<item name="Tycho CI Friendly Versions" href="TychoCiFriendly.html" />
+			<item name="Creating update sites using category.xml" href="Category.html" />
 			<item name="Target Platform Configuration" href="target-platform-configuration/plugin-info.html" />
 			<item name="Compiler Plugin" href="tycho-compiler-plugin/plugin-info.html" />
 			<item name="Declarative Services Plugin" href="tycho-ds-plugin/plugin-info.html" />