Add introduction to the architecture chapter.

Author: ehuss

src/doc/contrib/src/architecture/index.md

@@ -1 +1,8 @@
 # Architecture Overview
+
+This chapter gives a very high-level overview of Cargo's architecture. This is
+intended to give you links into the code which is hopefully commented with
+more in-depth information.
+
+If you feel something is missing that would help you, feel free to ask on
+Zulip.