Documentation/x86: Document TDX kernel architecture

Document the TDX guest architecture details like #VE support,
shared memory, etc.

[ dhansen: made some wording changes, including removing
	   all the plural "#VE's" and "#VEs". ]

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lkml.kernel.org/r/20220405232939.73860-31-kirill.shutemov@linux.intel.com