ref-manual: add numa to machine features

numa is an existing machine feature, add it to the list so that users
are aware of it.

(From yocto-docs rev: 6022d15b8ee5e038d2f1c0f061a871dba40e5213)

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Reviewed-by: Quentin Schulz <foss+yocto@0leil.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/features.rst

@@ -62,6 +62,8 @@ Project metadata:
 
 -  *keyboard:* Hardware has a keyboard
 
+-  *numa:* Hardware has non-uniform memory access
+
 -  *pcbios:* Support for booting through BIOS
 
 -  *pci:* Hardware has a PCI bus