PIPE-3211: fold requirements into `pyproject.toml` and add CASA dependency installer
- Declare dependencies and optional extras (dev, docs, exp) statically in `pyproject.toml` per PEP 621, removing dynamic dependency file tables.
- Remove `requirements*.txt` and legacy `Dockerfile`.
- Add `scripts/install_dependencies.py` to allow installing dependencies into monolithic CASA environments without installing Pipeline.
- Update `MANIFEST.in` to include `scripts/install_dependencies.py` in sdists.
- Update developer and setup documentation to reflect `pyproject.toml` as SSoT.
