Avoid `black` docs dependency

## What happened?

While working on (#3497) I've dived into the mkdocstrings/zensical docs and tried out lots of configuration.

So I come bearing confusion on this old comment 😅 (https://github.com/narwhals-dev/narwhals/pull/1608)

https://github.com/narwhals-dev/narwhals/blob/07b5d94db70e79cbc96c2e08ed6d2c497248174a/pyproject.toml#L107-L108


Well, it turns out that support for `ruff` was merged **2 days later** in (https://github.com/mkdocstrings/python/pull/216) and released shortly after in [`1.13.0`].

> [!IMPORTANT]
> All of this was from 2024

## The fix?
We'd need to remove `black` from the `docs` group, since `mkdocstrings` [will pick it first if installed].


## Related
The only other place I've seen `black` referenced is here:

<details><summary>Show <code>pre-commit-config.yaml</code></summary>
<p>

https://github.com/narwhals-dev/narwhals/blob/07b5d94db70e79cbc96c2e08ed6d2c497248174a/.pre-commit-config.yaml#L115-L121

</p>
</details> 

1. Since that hook has inline dependencies declared, removing from `docs` might be a non-issue?
2. There are alternatives to `blacken-docs`, like 
    i. Ruff's (preview) [Markdown code formatting]
    ii. Zensical's [Snippets] extension ([more docs])

[`1.13.0`]: https://github.com/mkdocstrings/python/releases/tag/1.13.0
[will pick it first if installed]: https://github.com/mkdocstrings/python/blob/dc6aa93b1c956bc02904f33d1f6f635b275175d9/src/mkdocstrings_handlers/python/_internal/rendering.py#L508-L518
[Markdown code formatting]: https://docs.astral.sh/ruff/formatter/#markdown-code-formatting
[Snippets]: https://zensical.org/docs/setup/extensions/python-markdown-extensions/#snippets
[more docs]: https://facelessuser.github.io/pymdown-extensions/extensions/snippets/

[]