doc: add jlink plugin documentation

Add initial documentation for the jlink plugin.

docs/common/craft-parts/reference/plugins/jlink_plugin.rst

@@ -0,0 +1,88 @@
+.. _craft_parts_jlink_plugin:
+
+JLink plugin
+=============
+
+The JLink plugin can be used for Java projects where you would want to
+deploy Java runtime specific for your application or install a minimal
+Java runtime.
+
+
+Keywords
+--------
+
+This plugin uses the common :ref:`plugin <part-properties-plugin>` keywords as
+well as those for :ref:`sources <part-properties-sources>`.
+
+Additionally, this plugin provides the plugin-specific keywords defined in the
+following sections.
+
+jlink-java-version
+~~~~~~~~~~~~~~~~~~~
+**Type:** str
+
+Java package version to use for the build (e.g. 21 will install openjdk-21-jdk).
+
+jlink-jars
+~~~~~~~~~~~~~~~~~~
+**Type:** list of strings
+
+List of paths to jar files of your application. If not specified, plugin
+will find all jar files in the staging area.
+
+Dependencies
+------------
+
+By default this plugin uses openjdk-21-jdk from the build base.
+
+The user is expected to stage openjdk dependencies either by installing
+an appropriate openjdk slice:
+
+.. code-block:: yaml
+
+    parts:
+        runtime:
+            plugin: jlink
+            after:
+                - deps
+
+        deps:
+            plugin: nil
+            stage-packages:
+                - openjdk-21-jre-headless_security
+
+or by installing the dependencies directly:
+
+.. code-block:: yaml
+
+    parts:
+        runtime:
+            plugin: jlink
+            after:
+                - deps
+
+        deps:
+            plugin: nil
+            stage-packages:
+                - libc6_libs
+                - libgcc-s1_libs
+                - libstdc++6_libs
+                - zlib1g_libs
+                - libnss3_libs
+
+
+How it works
+------------
+
+During the build step, the plugin performs the following actions:
+
+* Finds all jar files in the staging area or selects jars specified in
+  ``jlink_jars``.
+* Unpacks jar files to the temporary location and concatenates all embedded jars
+  into `jdeps <jdeps_>`_ classpath.
+* Runs `jdeps <jdeps_>`_ to discover Java modules required for the staged jars.
+* Runs `jlink <jlink_>`_ to create a runtime image from the build JDK.
+
+
+.. _`jdeps`: https://docs.oracle.com/en/java/javase/21/docs/specs/man/jdeps.html
+.. _`jlink`: https://docs.oracle.com/en/java/javase/21/docs/specs/man/jlink.html