Update documentation about defining xdpcap map for libbpf >= 1.0.0

With libbpf 1.0.0 the expected way that a map is defined has changed. This means that users who wish to integrate xdpcap need to use a different way to define their maps than the one suggested in `hook.h`. Update documentation accordingly.

Start with hook.h and review the documented map-definition approach against the libbpf 1.0.0 change described in the issue. Update the documentation so xdpcap users have the current map-definition guidance, then verify that the outdated instructions are no longer present.

Written by the indexing model from the issue text.