docs: Fix file permissions for documentation

Many of the documentation files had executable permissions. This was
an error. Fix this.

Change-Id: I99b905b7325d2450dd655991c5e41e238c7fdd03
Signed-off-by: Stephen Finucane <stephen.finucane@intel.com>
Reviewed-by: Mark D. Gray <mark.d.gray@intel.com>