doc: updated What is ACRN introduction

* Update and simplify our What is ACRN document.
* Removed SDC disussion and out-of-date best known configuration
  descriptions and scenario block diagrams
* Removed I/O Mediator discussion from "What is ACRN" and move
  referenced label to content in the HLD overview (that was duplicated in
  "What is ACRN")
* Updated images for scenarios and removed obsolete images
* Add references to new overview docs and GSG
* Add general description of VMs in example scenarios

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/introduction/images/boot-flow-2.dot

@@ -1,7 +1,7 @@
 digraph G {
    rankdir=LR;
    bgcolor="transparent";
-   UEFI -> "GRUB" -> "acrn.32.out" -> "Pre-launched\nVM Kernel"
-     "acrn.32.out" -> "Service VM\nKernel" -> "ACRN\nDevice Model" ->
+   UEFI -> "GRUB" -> "acrn.bin" -> "Pre-launched\nVM Kernel"
+     "acrn.bin" -> "Service VM\nKernel" -> "ACRN\nDevice Model" ->
 "Virtual\nBootloader";
 }

doc/introduction/images/boot-flow.dot

@@ -1,6 +0,0 @@
-digraph G {
-   rankdir=LR;
-   bgcolor="transparent";
-   UEFI -> "acrn.efi" -> "OS\nBootloader" ->
-     "SOS\nKernel" -> "ACRN\nDevice Model" -> "Virtual\nBootloader";
-}