Design pattern index page (#11468)

_Description of what this PR is changing or adding, and why:_

This PR adds an 'index' page for the Design Pattern cookbook recipes
under App Architecture in the side nav.
* Adds the page
* Adds a `design-pattern.yml` file with 
* Adds an "Expansion Panel List" component (html + css)

**This is still very much a work in progress.** Before merging:
- [x] Land #11444 + add them to the data file, and create icons for
their rows
- [x] Update the `design-pattern.yml` with real data
- [x] Create the remaining needing icons (right now all four list items
use the same icon)
- [x] Replace 'LOREM IPSUM, BROTHER' 😅 

_Issues fixed by this PR (if any):_

* This will resolve #11374

---------

Co-authored-by: Parker Lougheed <[email protected]>

Author: ericwindmill

diagrams/app-architecture/architecture-docs.fig

@@ -10,7 +10,7 @@ dependencies:
 
 dev_dependencies:
   example_utils:
-    path: ../../../example_utils
+    path: ../../example_utils
 
 flutter:
   uses-material-design: true