Migrate markdown documentation to javadoc

The markdown documentation should be migrated to javadoc. This tightly couples the documentation and the source code, which has numerous advantages: 1. It is immediately visible to the user of the Smack API and 2. it reduces the chances that the documentation and the actual implementation diverge.

This also includes migrating the sinttest documentation to javadoc. Hence we should probably also publish the results of smack-integration-test:javadoc to make it easily accessible.