selftests: Move networking/timestamping from Documentation

Remove networking from Documentation Makefile to move the test to
selftests. Update networking/timestamping Makefile to work under
selftests. These tests will not be run as part of selftests suite
and will not be included in install targets. They can be built and
run separately for now.

This is part of the effort to move runnable code from Documentation.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>