docs(book): explain the basic relations in an Akita fold

book/src/SUMMARY.md

@@ -22,6 +22,7 @@
   - [Transcript and instance binding](./how/transcript.md)
   - [The proving protocol](./how/proving/proving.md)
     - [Field-to-ring evaluation reduction](./how/proving/field-ring-reduction.md)
+    - [Basic relations in an Akita fold](./how/proving/akita-fold.md)
     - [Opening points and digit-innermost layout](./how/proving/opening-points-layout.md)
     - [Fold path and field geometry](./how/proving/fold-path.md)
     - [Root fold and ring switching](./how/proving/root-fold-ring-switch.md)