Doc: networking: Fix the title's Sphinx overline in rds.rst

A missing "=" caused the title and sections to not show up properly in
the htmldocs.

Signed-off-by: Ahmed Zaki <anzaki@gmail.com>
Link: https://lore.kernel.org/r/20211128171719.3286255-1-anzaki@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

docs: networking: convert rds.txt to ReST

- add SPDX header;
- add a document title;
- mark code blocks and literals as such;
- mark tables as such;
- mark lists 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>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>