Documentation/x86: Add documentation for TDX host support

Add documentation for TDX host kernel support.  There is already one
file Documentation/x86/tdx.rst containing documentation for TDX guest
internals.  Also reuse it for TDX host kernel support.

Introduce a new level menu "TDX Guest Support" and move existing
materials under it, and add a new menu for TDX host kernel support.
Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20231208170740.53979-19-dave.hansen%40intel.com