Documentation: Add a CartesianConfigTricks documentation

This is useful for people wanting to execute tests on
a different order than the one they appear in the cartesian
config file. This could eventually grow to add more tips
and tricks involving the cartesian config.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #1927 from autotest/cartesian_config_tricks_docs

Documentation: Add a CartesianConfigTricks documentation