docs: acrn: Introduce ACRN

docs: acrn: Introduce ACRN

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 <dave.hansen@intel.com>
Cc: Sen Christopherson <sean.j.christopherson@intel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Fengwei Yin <fengwei.yin@intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Yu Wang <yu1.wang@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Zhi Wang <zhi.a.wang@intel.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Shuo Liu <shuo.a.liu@intel.com>
Link: https://lore.kernel.org/r/20210207031040.49576-2-shuo.a.liu@intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>