Some overview content for Doxygen documentation.

Some overview content for Doxygen documentation.

Make the main page more useful by providing some explanation of the
purpose of the documentation and adding a (currently very short) list of
links to more detailed pages, making it easier to start consuming the
documentation.

Added a page that contains some information from the wiki
http://www.gromacs.org/Developer_Zone/Programming_Guide/Library_Structure

Once the documentation is more complete, some wiki pages (including the
above) could go away.

Requires Doxygen >=1.8, because separate Markdown pages are used.

Change-Id: I2bf6e00976b47b0d34c79f4ea09452d40701f753