avutil: Move library version related macros to version.h

This is a more sensible place for these macros.
doxy: Fix link in badge color

Green on blue was not exactly optimal.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
doxy: Update the css to have a flat style

Drop references to the doxy image gradients and style a the code blocks to
keep the whitespace and indent properly.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>