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)
