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>

CODING_STYLE

@@ -14,6 +14,12 @@ explicitly (e.g. tools/libxl/CODING_STYLE) but often implicitly (Linux
 coding style is fairly common). In general you should copy the style
 of the surrounding code. If you are unsure please ask.
 
+MISRA C
+-------
+
+The Xen Hypervisor follows some MISRA C coding rules. See
+docs/misra/rules.rst for details.
+
 Indentation
 -----------