docs: Introduce Fusa Requirement and define maintainers

The FUSA folder is expected to contain requirements and other documents
to enable safety certification of Xen hypervisor.
Added a intro to explain how the requirements are categorized, written
and their supported status.
Also, added index.rst for inclusion in build docs.

Added maintainers for the same.

Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Acked-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Michal Orzel <michal.orzel@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

MAINTAINERS

@@ -315,6 +315,15 @@ F:	xen/arch/x86/include/asm/x86_*/efi*.h
 F:	xen/common/efi/
 F:	xen/include/efi/
 
+FUSA
+M:	Stefano Stabellini <sstabellini@kernel.org>
+M:	Bertrand Marquis <bertrand.marquis@arm.com>
+M:	Michal Orzel <michal.orzel@amd.com>
+M:	Ayan Kumar Halder <ayan.kumar.halder@amd.com>
+M:	Artem Mygaiev <artem_mygaiev@epam.com>
+S:	Supported
+F:	docs/fusa/
+
 GDBSX DEBUGGER
 M:	Elena Ufimtseva <elena.ufimtseva@oracle.com>
 S:	Supported