Reorganizing the doc.rs documentation

[Pat-Lafon]

As noted in #420, the current https://docs.rs/z3/latest/z3/ is very flat. This makes it both challenging to understand what one is looking for, and for including examples(like in using Z3 datatypes which has several important data structures and functions spread over multiple pages instead of having a centralized datatype module (https://docs.rs/z3/latest/z3/datatype_builder/index.html) with examples).

[toolCHAINZ]

Not a reorganization, but I made an initial try at providing a simple top-level example in the crate docs in #437.