Update Doxygen config for generating online API docs

Enabled implicit brief descriptions (JAVADOC_AUTOBRIEF), and markdown support (MARKDOWN_SUPPORT) for doc blocks. This means that we no longer need to explicitly inclufe a @brief tag in the doc block, the first full sentence will be regarded as the brief documentation if it's not explicitly given. Also we can use Markdown formatting in the comments, which is a bit nicer than the native Doxygen tags.

I also disabled the Doxygen_phpvarfilter, but leave it commented out. It should not be needed anymore unless somebody is using an ancient version of doxygen. (Don't do that!)

I also changed the heading a bit, removed "The" from "The Hubzilla", and added a tagline. Feel free to revise to whatever conforms to the project norms.