bpf, docs: Move handling of maps to Documentation/bpf/maps.rst

Move the general maps documentation into the maps.rst file from the
overall networking filter documentation and add a link instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211119163215.971383-5-hch@lst.de