Add documentation on the following aspects of ACRN:

1) A brief introduction on the architecture of ACRN.
2) I/O request handling in ACRN.
3) CPUID functions of ACRN.

To learn more about ACRN, please go to ACRN project website
https://projectacrn.org, or the documentation page
https://projectacrn.github.io/.

Cc: Dave Hansen
Cc: Sen Christopherson
Cc: Dan Williams
Cc: Fengwei Yin
Cc: Zhi Wang
Cc: Zhenyu Wang
Cc: Yu Wang
Cc: Reinette Chatre
Cc: Greg Kroah-Hartman
Cc: Randy Dunlap
Reviewed-by: Zhi Wang
Reviewed-by: Reinette Chatre
Reviewed-by: Borislav Petkov
Signed-off-by: Shuo Liu
Link: https://lore.kernel.org/r/20210207031040.49576-2-shuo.a.liu@intel.com
Signed-off-by: Greg Kroah-Hartman