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

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

book/src/values.md

@@ -1,10 +1,14 @@
 # POD value types
-From the frontend perspective, POD values may be one of three[^type] types:
+From the frontend perspective, POD values may be one of the following[^type] types: two atomic types
 - `Integer`
 - `String`
-- `Dictionary`, `array`, `set`
 
-From the backend perspective, however, these types will all be encoded as a fixed number of field elements, the number being chosen so as to accommodate the `Integer` type as well as hashes to represent the `String` and `MerkleTree` types with the appropriate level of security.
+and three compound types
+- `Dictionary`
+- `Array`
+- `Set`.
+
+From the backend perspective, however, these types will all be encoded as a fixed number of field elements, the number being chosen so as to accommodate the `Integer` type as well as hashes to represent the `String` and compound types with the appropriate level of security.
 
 In the case of the Plonky2 backend with 100 bits of security, all of these types are represented as 4 field elements, the output of the Poseidon hash function used there being