doc: some improvements

Signed-off-by: Ran Benita <ran234@gmail.com>
doc: update Doxyfile template to one from newer version

We want to use the USE_MDFILE_AS_MAINPAGE option which was introduced in
doxygen 1.8.3 (released 2012-12).

Right now the new options are commented, otherwise older doxygen spews
these these warnings, which can be ignored:

warning: ignoring unsupported tag `USE_MDFILE_AS_MAINPAGE =' at line 794, file doc/Doxyfile
warning: ignoring unsupported tag `MATHJAX_FORMAT         =' at line 1210, file doc/Doxyfile
warning: ignoring unsupported tag `EXTERNAL_SEARCH        =' at line 1257, file doc/Doxyfile
warning: ignoring unsupported tag `SEARCHENGINE_URL       =' at line 1265, file doc/Doxyfile
warning: ignoring unsupported tag `SEARCHDATA_FILE        =' at line 1271, file doc/Doxyfile
warning: ignoring unsupported tag `EXTERNAL_SEARCH_ID     =' at line 1278, file doc/Doxyfile
warning: ignoring unsupported tag `EXTRA_SEARCH_MAPPINGS  =' at line 1287, file doc/Doxyfile

Signed-off-by: Ran Benita <ran234@gmail.com>
doc: use README as doxygen main page overview

The doxygen page looked a bit dead, the README fills it nicely, and is
already written in the markdown format which doxygen uses (I think?).

Unfortunately the USE_MDFILE_AS_MAINPAGE doxygen config doesn't seem to
do anything.. So we just add a {#mainpage} tag at the top of the README
which isn't so bad. BUT we still need some config option (the
no_extension=md part) so that doxygen will accept README instead of
README.md or somesuch. And that requires an even newer release, 1.8.3.1,
released 2013-01. But if an older version is used, it doesn't spew out
warnings but just skips the README, which is fine.

Signed-off-by: Ran Benita <ran234@gmail.com>