Migrate markdown documentation to javadoc

While markdown is easier to write, Smack's markdown documentation was
never tightly coupled with the source. For example, the markdown
documentation never provided links to the actual Java classes and
methods. This poses the risk that the documentation and the code
diverge over time. Furthermore, javadoc is constantly improving (for
example @snippet annotations) and I expect that one will be able to
write javadoc in markdown.

Fixes SMACK-928.

smack-java8-full/src/main/java/org/jivesoftware/smackx/SmackExtensions.java

@@ -0,0 +1,26 @@
+/**
+ *
+ * Copyright 2022 Florian Schmaus
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.jivesoftware.smackx;
+
+/**
+ * This is just a dummy class, please head over to {@link org.jivesoftware.smackx} for more information on Smack
+ * extensions. The dummy class causes javadoc generate the HTML for smackx.pacakge-info.java, which would otherwise be
+ * not generated, as org.jivesoftware.smackx is an empty package (see
+ * <a href="https://bugs.openjdk.org/browse/JDK-4492654">JDK-4492654</a>).
+ */
+public class SmackExtensions {
+}