[doc] Create empty class so that javadoc jar is created

This needs to be done to fulfill the requirement of maven
central, that every artefact has to have a javadoc jar.
However, pmd-doc only contains internal classes.

pmd-doc/src/main/java/net/sourceforge/pmd/doc/InternalApiBridge.java

@@ -0,0 +1,25 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+package net.sourceforge.pmd.doc;
+
+import net.sourceforge.pmd.annotation.InternalApi;
+
+/**
+ * Internal API.
+ *
+ * <p>Acts as a bridge between outer parts of PMD and the restricted access
+ * internal API of this package.
+ *
+ * <p><b>None of this is published API, and compatibility can be broken anytime!</b>
+ * Use this only at your own risk.
+ *
+ * @apiNote Internal API
+ */
+@InternalApi
+public final class InternalApiBridge {
+    // note: this is empty - all classes in pmd-doc are internal.
+    // this class is only here to fulfill the maven central requirement,
+    // that every artefact has to have a javadoc jar.
+}

pmd-doc/src/main/java/net/sourceforge/pmd/doc/package-info.java

@@ -0,0 +1,8 @@
+/*
+ * BSD-style license; for more info see http://pmd.sourceforge.net/license.html
+ */
+
+/**
+ * This module contains only internal code to generate the rule documentation pages.
+ */
+package net.sourceforge.pmd.doc;