flow_dissector: rst'ify documentation

Rename bpf_flow_dissector.txt to bpf_flow_dissector.rst and fix
formatting. Also, link it from the Documentation/networking/index.rst.

Tested with 'make htmldocs' to make sure it looks reasonable.

Fixes: ae82899b

 ("flow_dissector: document BPF flow dissector environment")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>