doc: Add information on Twister Statuses

Currently, Twister Statuses are only useful for
the initiated, save for the very basics.
This is in opposition to the fact
that they are the main thing end users take into
account when checking their Twister run reports.

In order to make Statuses more useful for the end user,
a new documentation page has been created,
elucidating the full meaning of all Statuses.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>