Documentation: hid: intel-ish-hid: add section for firmware loading

Add a section to describe the ISH firmware loading process for Lunar Lake
and later generations.

Signed-off-by: Qianru Huang <qianru.huang@intel.com>
Signed-off-by: Zhang Lixu <lixu.zhang@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>