Organize docs: front and back end; custom predicates. (#96)

* Organize docs: front and back end; custom predicates.

* Whoops forgot to hit save before git commit last time -- delete stuff moved out of values.md

* Update book/src/values.md

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

book/src/customhash.md

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+# How to hash a custom predicate
+
+Every predicate, native or custom, is identified on the backend by a predicate ID.
+
+The native predicates are numbered with small integers, sequentially.  The ID of a custom predicate is a hash of its definition; this guarantees that two different predicates cannot have the same ID (aside from the miniscule probability of a hash collision).
+
+This document explains in some detail how the definition of a custom predicate is serialized and hashed.
+
+Custom predicates are defined in _groups_ (also known as _batches_); see an [example](./customexample.md).  The definition of a custom predicate in a group involves other predicates, which may include:
+- native predicates
+- previously-defined custom predicates
+- other predicates in the same group.
+
+Predicate hashing is recursive: in order to hash a group of custom predicates, we need to know IDs for all the previously-defined custom predicates it depends on.
+
+The definition of the whole group of custom predicates is serialized (as explained below), and that serialization is hashed (using a zk-friendly hash -- in the case of the plonky2 backend, Poseidon) to give a _group ID_.  Each predicate in the group is then referenced by
+```
+predicate_ID = (group_ID, idx)
+```
+(here `idx` is simply the index of the predicate in the group).