Fix broken link (#593)

I noticed the link-checker to failed (see
[here](https://github.com/UCL-ARC/python-tooling/actions/runs/18410874282/job/54005605441)).

```
* [404] <https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/> | Rejected status code (this depends on your "accept" configuration): Not Found
```

This PR replaces the link with a link to
`https://packaging.python.org/en/latest/guides/writing-pyproject-toml/`,
which fits the description in the documentation.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: stellaprins

docs/pages/packaging.md

@@ -45,7 +45,7 @@ a higher-level package which is effectively the same, but much easier to reuse.
 
 | Name                                                                                               | Short description                                                                                                                                                  | 🚦                                           |
 | -------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ | -------------------------------------------- |
-| [pyproject.toml](https://pip.pypa.io/en/stable/reference/build-system/pyproject-toml/)             | Contains build system requirements and information, which are used by pip to build the package. It is becoming the accepted standard and we strongly recommend it. | <span class="label label-green">Best</span>  |
+| [pyproject.toml](https://packaging.python.org/en/latest/guides/writing-pyproject-toml/)            | Contains build system requirements and information, which are used by pip to build the package. It is becoming the accepted standard and we strongly recommend it. | <span class="label label-green">Best</span>  |
 | [setup.py](https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/)  | Strongly coupled with setuptools and therefore not recommended.                                                                                                    | <span class="label label-yellow">Good</span> |
 | [setup.cfg](https://packaging.python.org/en/latest/guides/distributing-packages-using-setuptools/) | An ini file that contains defaults for setup.py commands.                                                                                                          | <span class="label label-yellow">Good</span> |