libbpf: Improve libbpf API documentation link position

This puts the link for libbpf API documentation into the sidebar
for much easier navigation.

You can preview this change at:

  https://libbpf-test.readthedocs.io/en/latest/

Note that the link is hardcoded to the production version, so you
can see that it self references itself here for now:

  https://libbpf-test.readthedocs.io/en/latest/api.html



This will need to make its way into the libbpf mirror, before being
deployed to libbpf.readthedocs.org

Signed-off-by: Grant Seltzer <grantseltzer@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220422031050.303984-1-grantseltzer@gmail.com