CONTRIBUTING: Improve and update formatting docs

Names of files and directories should be in lowercase, with dashes between words — not in camel case. For instance, it should be `all-packages.nix`, not `allPackages.nix` or `AllPackages.nix`.

### Formatting

CI [enforces](./.github/workflows/check-nix-format.yml) all Nix files to be

formatted using the [official Nix formatter](https://github.com/NixOS/nixfmt).

You can ensure this locally using either of these commands:

```

nix-shell --run treefmt

nix develop --command treefmt

nix fmt

```

If you're starting your editor in `nix-shell` or `nix develop`,

you can also set it up to automatically format the file with `treefmt` on save.

If you have any problems with formatting, please ping the

[formatting team](https://nixos.org/community/teams/formatting/) via

[@NixOS/nix-formatting](https://github.com/orgs/NixOS/teams/nix-formatting).

### Syntax

- Set up [editorconfig](https://editorconfig.org/) for your editor, such that [the settings](./.editorconfig) are automatically applied.

- Use `lowerCamelCase` for variable names, not `UpperCamelCase`. Note, this rule does not apply to package attribute names, which instead follow the rules in [package naming](./pkgs/README.md#package-naming).

- New files must be formatted by entering the `nix-shell` from the repository root and running `nixfmt`.

- Functions should list their expected arguments as precisely as possible. That is, write

  ```nix