boards/intel_adsp_cavs25: Add board documentation

The default development environment for this target is in fact a
consumer Chromebook available to anyone.  So it deserves public
documentation.

Note that this spends considerably more time explaining the details of
how to configure a chromebook as a Zephyr development platform than it
does the process for building and running Zephyr (which is really very
conventional).  Chromebooks, which allow user-signed audio firmware,
are a great boon to SOF development.  But they were never intended as
developer devices themselves.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>