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.
