build: migrate development workflow to uv - #800

Moves the development workflow to uv. The build backend (setuptools + setuptools-scm) and the tox envs stay as they are.

Dependencies

• Dependencies live in PEP 735 [dependency-groups] in pyproject.toml (test, docs, lint, typing, tox, and dev which includes them) and are pinned in uv.lock.
• tox installs from those groups through tox-uv instead of per-env deps. The style env skips installing the package, since ruff doesn't need it.
• requirements.txt, requirements.test.txt and doc/requirements.txt are removed.
• Dependabot uses the uv ecosystem for the root, the /doc pip entry is dropped, and the dead docs link in its header is fixed.

CI

• pythonpackage.yml, pythonpackage-windows.yml, style.yml and documentation.yml are merged into a single ci.yml. The test matrix runs on Ubuntu and Windows for Python 3.11 to 3.15, and lint and the docs build run once, on Ubuntu with Python 3.13.
• CI sets up uv with astral-sh/setup-uv (pinned to a commit SHA) and runs tox via uv run --locked --only-group tox tox, so the tox versions come from uv.lock.
• Read the Docs installs with its native uv support (python.install with method: uv, the docs group and the all extra).

Docs

CONTRIBUTING.rst gets a short development setup section.

Notes for reviewers

• The job names change (checks (<os>, <python>)), and style and docs are no longer separate checks, so any required status checks in the branch protection settings need updating.
• style and docs now run on pushes to master and on PRs only, not on every push.

Closes #704

Testing

Locally: tox -e style, tox -e mypy and tox -e 3.13. All checks pass in CI on the latest commit, including the Windows jobs, the docs build and the Read the Docs build.