Code quality/project standards alignment

This is a meta-issue, intended to track sub-issues that will align Slither's codebase, development practices, etc. with the rest of our public tooling.

- [ ] Align `README.md` with our doc practices
- [ ] Streamline our installation steps
- [ ] Move developer-specific installation instructions to their own `CONTRIBUTING.md` (this file already exists, but doesn't explain how to get a development environment stood up)
- [ ] Evaluate simplifying the `pip` based installation (can we depend on `solc` directly?)
- [ ] Modernize the Python codebase
- [ ] Switch to `pyproject.toml`
- [ ] Use `pypa/build` for the build (sdist and wheel(s))
- [ ] Use `ruff`?
- [ ] Track Python dependency upgrades via `dependabot.yml`
- [ ] Add some kind of `QUICKSTART.md` or similar guide

(This list is not intended to be exhaustive, and will be updated over time!)