feat: updating CONTRIBUTING.md to reflect switch from black to ruff.

-   Reorganize sections.
-   Rewrite chunks of prose.
-   Replace mentions of `black` with mentions of `ruff`.
-   And describe `uv` usage.

Note: `CONTRIBUTING.md` refers to `pre-commit`,
and the old `requires-optional.txt` installed it,
but there isn't a `.pre-commit-config.yaml` file in this repository.
Wuzzup?

Author: gvwilson