Add a section on Kani attributes to the docs

The docs mention the Kani attributes, e.g. `kani::proof` and `kani::unwind` in the tutorial, but there is no dedicated section that lists all the available attributes. We should add one (to Getting Started?) that includes:

- [ ] `kani::proof`
- [ ] `kani::unwind`
- [ ] `kani::solver`
- [ ] `kani::stub`