Documentation/x86: Update IOMMU spec references to use stable identifiers

Documentation/x86: Update IOMMU spec references to use stable identifiers

Direct URLs to vendor specifications for Intel VT-d and AMD IOMMU
are frequently changed by vendors, leading to broken links in the
documentation.

Replace the fragile URLs with persistent identifiers, providing the
official document titles and IDs. This ensures users can locate the
relevant specifications regardless of vendor website restructuring.

Signed-off-by: Richard Lyu <richard.lyu@suse.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://patch.msgid.link/20260106052815.46114-1-richard.lyu@suse.com