book: Reorganize design subsections

book/src/SUMMARY.md

@@ -13,9 +13,11 @@
   - [Gadgets](user/gadgets.md)
   - [Tips and tricks](user/tips-and-tricks.md)
 - [Design](design.md)
-  - [Multipoint opening argument](design/multipoint-opening.md)
-  - [Permutation argument](design/permutation.md)
-  - [Lookup argument](design/lookup-argument.md)
+  - [Proving system](design/proving-system.md)
+    - [Multipoint opening argument](design/proving-system/multipoint-opening.md)
+    - [Permutation argument](design/proving-system/permutation.md)
+    - [Lookup argument](design/proving-system/lookup-argument.md)
+  - [Implementation](design/implementation.md)
   - [Gadgets](design/gadgets.md)
     - [SHA-256](design/gadgets/sha256.md)
       - [16-bit table chip](design/gadgets/sha256/table16.md)

book/src/design/implementation.md

@@ -0,0 +1 @@
+# Implementation

book/src/design/proving-system.md

@@ -0,0 +1 @@
+# Proving system