docs: Add ARCHITECTURE.md

It's a common idea to have ARCHITECTURE.md next to README.md and
CONTRIBUTING.md [1].
So, let's do so instead of having the doc only under mdbook.

[1] https://matklad.github.io//2021/02/06/ARCHITECTURE.md.html

BUG=b:195003973
TEST=mdbook build

Change-Id: I8d3f479ba65c7f85865c51b5c22fb5b37d6e8f78
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3188675
Commit-Queue: Dennis Kempin <denniskempin@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
Reviewed-by: Dennis Kempin <denniskempin@google.com>

ARCHITECTURE.md

@@ -1,4 +1,4 @@
-# Architectural Overview (last edit: January 21, 2020)
+# Architecture
 
 The principle characteristics of crosvm are:
 

docs/book/src/SUMMARY.md

@@ -9,13 +9,13 @@
     -   [System Requirements](./running_crosvm/requirements.md)
     -   [Features](./running_crosvm/features.md)
     -   [Devices](./running_crosvm/devices.md)
+-   [Architecture](./architecture.md)
 -   [Contributing](./contributing.md)
 
 --------------------------------------------------------------------------------
 
 
 -   [Appendix](./appendix/index.md)
-    -   [Architectural Overview](./appendix/architectural_overview.md)
     -   [Example Usage (Outdated)](./appendix/example_usage.md)
     -   [Sandboxing](./appendix/sandboxing.md)
     -   [Seccomp](./appendix/seccomp.md)

docs/book/src/architecture.md

@@ -0,0 +1 @@
+{{#include ../../../ARCHITECTURE.md}}