The headers and footers are used to integrate Doxygen into Libav's website.
Fixes the Libav-ish look when doing `make apidoc`.
Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Documentation includes only the externally visible API of the installed
headers.

Based on a patch by Anton Khirnov <anton@khirnov.net>.
Signed-off-by: Anton Khirnov <anton@khirnov.net>