docs: networking: device drivers: convert amazon/ena.txt to ReST

- add SPDX header;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- mark tables 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/device_drivers/index.rst

@@ -29,6 +29,7 @@ Contents:
    stmicro/stmmac
    3com/3c509
    3com/vortex
+   amazon/ena
 
 .. only::  subproject and html
 

MAINTAINERS

@@ -815,7 +815,7 @@ R:	Saeed Bishara <saeedb@amazon.com>
 R:	Zorik Machulsky <zorik@amazon.com>
 L:	netdev@vger.kernel.org
 S:	Supported
-F:	Documentation/networking/device_drivers/amazon/ena.txt
+F:	Documentation/networking/device_drivers/amazon/ena.rst
 F:	drivers/net/ethernet/amazon/
 
 AMAZON RDMA EFA DRIVER