HID: THC: Add documentation

Add Documentation/hid/intel-thc-hid.rst file to provide hardware
and software detail for intel THC drivers.

Co-developed-by: Sun Xinpeng <xinpeng.sun@intel.com>
Signed-off-by: Sun Xinpeng <xinpeng.sun@intel.com>
Signed-off-by: Even Xu <even.xu@intel.com>
Reviewed-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Tested-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>