Merge changes to docs (#41)

* Merge changes to docs

* Fix typo

* Correct SUMMARY so it compiles; update .gitignore

* Clean up statements.md

Make syntax and notation consistent with Rust source code.

* Fix statements for Merkle trees and compound types

* First draft of custom statements and small updates to signedpod.md

* Update book/src/merkletree.md

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* merklestatements correct typo

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* add todo for gadget ids

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* Remove custom statements, will do on separate branch

* Restore Merkle examples and statements table

---------

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

book/src/SUMMARY.md

@@ -3,14 +3,18 @@
 - [Introduction](./introduction.md)
 
 # Specification
-- [Backend types](./backendtypes.md)
-	- [MerkleTree](./merkletree.md)
+- [The frontend structure of a POD]()
+  - [Frontend POD value types](./values.md)
+  - [Anchored keys](./anchoredkeys.md)
+- [The backend structure of a POD]()
+  - [Backend types](./backendtypes.md)
+  - [MerkleTree](./merkletree.md)
 - [Deductions](./deductions.md)
-	- [Statements](./statements.md)
-	- [Operations](./operations.md)
+  - [Statements](./statements.md)
+    - [Statements involving compound types and Merkle trees](./merklestatements.md)
+  - [Operations](./operations.md)
 - [POD types](./podtypes.md)
-	- [SignedPOD](./signedpod.md)
-	- [MainPOD](./mainpod.md)
-	- [MockPOD](./mockpod.md)
-- [POD value types](./values.md)
-- [Examples](./examples.md)
+  - [SignedPOD](./signedpod.md)
+  - [MainPOD](./mainpod.md)
+  - [MockPOD](./mockpod.md)
+- [Examples](./examples.md)