Add sections to the user guide explaining witness optionality

[str4d]

One of the design choices we made in halo2_proofs is that we want circuit synthesis and witness generation logic to be closely-coupled, to reduce chances for error (a design choice we adopted from bellman). As a result, all witness values are assigned via closures that are not executed during keygen (when we have no witnesses), and passed around as Option<T> (and are None during keygen). We then have gadgets that are meant to encapsulate this optionality, allowing the developers to only need to deal with it at the edges of their circuit logic.

We don't currently explain this well. In one example, a user misread the Orchard circuit logic they were using as an example, and tried making the gadgets in their circuit optional (which does not work, as it changes the circuit structure). It probably didn't help that the in-circuit gadgets are often named the same as their outside-circuit equivalents that are used for witnesses (e.g. the gadgets::ecc::Point gadget which can be instantiated from an Option<pallas::Point> witness).

We should add several new sections to the user guide:

• A section that explains why witnesses need to be optional.
• A tutorial on how to use gadgets, that includes an explanation of how we use them to wrap witness optionality.

[str4d]

See also #509 (comment) where I describe an alternate API that would make this less of a problem.

[str4d]

I've implemented the alternate API in #598, which should make the guide sections slightly easier to write.