Add `dummy.cc`, loosely based on `backends/functional/test_generic.cc` but as an actualy backend and without the memory testing.
Skeleton section for minimal functional backend, describing the aforementioned `dummy.cc`.
As the intro paragraph (now) says:
> This section will introduce the SMT-LIB functional backend and what changes are needed...
The example is intended to be read without prior knowledge of the SMT-LIB backend, but the previous version glossed over a lot and instead focused on *just* what was changed.
This version should now be easier to follow without prior knowledge, while still being able to learn enough about the `Smt` version to adapt it to a different s-expression target that isn't Rosette.
Also adds a few `literalinclude`s of smtlib.cc, which is now copied to `docs/source/generated` along with producing the rosette diff on the fly (which now also has up to 20 lines of context, enabling the full `Module::write()` diff to be literal included).
