Refactor, with no behavior change: move `scientific()` and `metric()` out of src/humanize/number.py into a new module src/humanize/notation.py, together with the helpers and constants only they use.

- `humanize.scientific`, `humanize.metric`, `humanize.number.scientific` and `humanize.number.metric` must keep working.
- Helpers still needed by number.py must not be duplicated.
- The new module must appear in the API documentation (docs/ and mkdocs.yml) like the other modules.
- Do not modify the existing tests; the whole test suite must pass.

The project virtualenv is ready in .venv: run the tests with `.venv/bin/python -m pytest -q`. Work only inside this repository. Do not commit.
