Group logic-related things under one section

I found the layout to be a bit inconsistent before. This groups anything
that touches logic rules under "Lowering to logic".

This might be crowding too many things under that section, but it makes
more sense to me overall.

Author: tmandry

src/SUMMARY.md

@@ -38,12 +38,12 @@
       - [Goals and clauses](./traits/goals-and-clauses.md)
       - [Equality and associated types](./traits/associated-types.md)
       - [Implied bounds](./traits/implied-bounds.md)
+      - [Well-formedness checking](./traits/wf.md)
       - [Region constraints](./traits/regions.md)
+      - [The lowering module in rustc](./traits/lowering-module.md)
+      - [Lowering rules](./traits/lowering-rules.md)
     - [Canonical queries](./traits/canonical-queries.md)
       - [Canonicalization](./traits/canonicalization.md)
-    - [Lowering rules](./traits/lowering-rules.md)
-      - [The lowering module in rustc](./traits/lowering-module.md)
-    - [Well-formedness checking](./traits/wf.md)
     - [The SLG solver](./traits/slg.md)
     - [An Overview of Chalk](./traits/chalk-overview.md)
     - [Bibliography](./traits/bibliography.md)