Documentation: Begin a RAS section

Add some initial RAS documentation. The expectation is for this to
collect, among others, all the user-visible features for interaction
with the RAS features of the kernel.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231128142049.GTZWX3QQTSaQk/+u53@fat_crate.local