Separate `simple_example` into "simple circuit" and "simple gadget"

[str4d]

https://zcash.github.io/halo2/user/simple-example.html renders https://github.com/zcash/halo2/blob/main/halo2_proofs/examples/simple-example.rs in the book to guide people in how to use halo2_proofs. However, it is really two examples in one:

• How to build a simple gadget and chip.
• How to construct a simple circuit.

We should split these into two separate examples in the book, and use existing gadgets for the latter. Note that this will require a separate example under halo2_gadgets, as we don't have access to that library of gadgets inside halo2_proofs.

[str4d]

Hmm, it might actually be better to write a separate "simple example", because halo2_gadgets does not (currently) have gadgets for arbitrary integer addition (only ECC math). So it would probably be better to write a separate example using one of the existing gadgets (e.g. Poseidon, or some of the ECC gadget).