Add Jest Architecture overview to docs. (jestjs#7449)

Author: cpojer

docs/Architecture.md

@@ -0,0 +1,8 @@
+---
+id: architecture
+title: Architecture
+---
+
+If you are interested in learning more about how Jest works, what the architecture behind the framework is, and how Jest is split up into individual reusable packages, check out this video:
+
+<iframe width="560" height="315" src="https://www.youtube.com/embed/3YDiloj8_d0" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>

website/sidebars.json

@@ -23,7 +23,8 @@
       "tutorial-jquery",
       "watch-plugins",
       "migration-guide",
-      "troubleshooting"
+      "troubleshooting",
+      "architecture"
     ],
     "Framework Guides": [
       "tutorial-react",