docs/misra: introduce rules.rst

docs/misra: introduce rules.rst

Introduce a list of MISRA C rules that apply to the Xen hypervisor. The
list is in RST format.

Specify that rules deviations need to be documented. Introduce a
documentation tag for in-code comments to mark them as deviations. Also
mention that other documentation mechanisms are work-in-progress.

Add a mention of the new list to CODING_STYLE.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Julien Grall <jgrall@amazon.com>