doc: networking: integrate scaling document into doc tree

Convert scaling document into reStructuredText and add reference to
scaling document into main table of contents in network documentation.

There are no semantic changes.

There are no references to "scaling.txt" file. Whole kernel tree was
checked using:
$ grep -r "scaling\.txt"
Signed-off-by: Otto Sabart <ottosabart@seberm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/networking/index.rst

@@ -34,6 +34,7 @@ Contents:
    snmp_counter
    checksum-offloads
    segmentation-offloads
+   scaling
 
 .. only::  subproject