Overhaul doxygen main page.

It got way too large, and contained quite a few links to .c files
whose doxygen page is not very interesting. For example, the doxygen
page of the .c files of filters, receivers and writers contain only
the init function since all other functions are static.

This commit limits the number of links on the main page considerably.
The new page tries to focus on important data structures and APIs
and does not strive for completeness.