docs: networking: convert packet_mmap.txt to ReST

This patch has a big diff, but most are due to whitespaces.

Yet, the conversion is similar to other files under networking:

- add SPDX header;
- add a document title;
- adjust titles and chapters, adding proper markups;
- mark lists as such;
- mark tables as such;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -89,6 +89,7 @@ Contents:
    nf_flowtable
    openvswitch
    operstates
+   packet_mmap
 
 .. only::  subproject and html