Documentation/x86/topology: Detail CPUID leaves used for topology enumeration

Add a new section describing the different CPUID leaves and fields used
to parse topology on x86 systems.

  [ bp: Cleanups and simplifications ontop. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/20250901170418.4314-1-kprateek.nayak@amd.com