This migrates page https://cvc4.github.io/sets-and-relations.
It further adds the SMT2 version of examples/api/cpp/sets.cpp and adds
test/regress/regress0/rels/relations-ops.smt2 as smtlib example for
relations.
This PR adds (most of) the existing examples to the documentation, and does some other minor updates on the documentation. Some details:
- for consistency, all cpp examples are moved from examples/api to examples/api/cpp
- add capabilities for SMT-LIB examples, and two simple smt2 examples
- more docs/examples/*.rst files
- two new documentation categories: installation (how to obtain, compile and install cvc5) and binary (about the cvc5 binary)