Documentation: Add debugfs docs with run after boot

Expand the documentation on the KUnit debugfs filesystem on the
run_manual.rst page.

Add section describing how to access results using debugfs.

Add section describing how to run tests after boot using debugfs.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>