Docs and notebooks point at DHI/ifm_contrib, which is private and archived

## The problem

The published documentation at https://dhi.github.io/ifm/ describes `ifm_contrib` as "the open source extension" and links to:

```
https://github.com/DHI/ifm_contrib/blob/master/doc/Notebooks/getting_started.ipynb
```

`DHI/ifm_contrib` is now **private and archived** (last push February 2023), so that link is a 404 for everyone reading the public site. It appears in `docs/tutorials.html` and in both legacy doc sets (7.4 and 7.5).

## It's not just a broken link

**10 of the 15 notebooks in this repo begin with `import ifm_contrib as ifm`:**

- `benchmarks/domenico_palciauskas/`
- `examples/adjust_error-tolerance-from-timeseries.ipynb`
- `examples/discrete-features-transient.ipynb`
- `examples/particle-tracing/find_layers_for_streamlines.ipynb`
- `extensions/open-loop/extension-open-loop.ipynb`
- `tutorials/mass-load-boundary-condition.ipynb`
- `tutorials/whats-new-in-ifm7400.ipynb`, `tutorials/whats-new-in-ifm7500.ipynb`
- `tutorials/zonebudget/create-zones.ipynb`, `tutorials/zonebudget/zone-budget-example.ipynb`

So `ifm_contrib` is on the critical path for most of the tutorial material, while the repository we point readers to is unreachable.

## Where the project actually lives now

The package is installable and current — `pip install ifm-contrib`, latest 0.2.4, released December 2024, MIT. Its PyPI metadata gives **Source Code** and **Bug Tracker** as `github.com/red5alex/ifm_contrib`, which is public, MIT, and actively maintained (last push February 2026, 6 open issues).

In other words, the maintained upstream sits outside the DHI organisation, and the copy inside the organisation is the archived one we link to.

## Also stale

`data/unittests/readme.md` says:

> these files are shared with the ifm_contrib unittest files. Please update in both repositories if changes are made.

That instruction can't be followed as written now that `DHI/ifm_contrib` is archived.

## Decision needed

This needs an owner rather than just a link fix, since there is no active maintainer for `ifm_contrib` inside DHI. Roughly:

1. **Repoint** — update the docs, the notebooks' install instructions and `data/unittests/readme.md` to the maintained location and to `pip install ifm-contrib`. Quickest, but leaves a DHI product tutorial depending on a personal repository.
2. **Bring it back in** — un-archive and make `DHI/ifm_contrib` public again, sync it with the maintained upstream, and take over publishing. More work, but puts the dependency of DHI's own tutorials back under DHI's control.
3. **Decouple** — rewrite the affected notebooks to use plain `ifm`, removing the dependency.

Raised from a hygiene sweep of the organisation's public repositories; see also #2, which adds the missing README and license here.

Start by reading docs/tutorials.html, the legacy 7.4 and 7.5 documentation, the ten listed notebooks, and data/unittests/readme.md to inventory references to DHI/ifm_contrib. Confirm whether the project will repoint, bring back, or decouple the dependency; done means all affected documentation, install instructions, notebooks, and README guidance match that decision and no longer rely on the archived repository.

Written by the indexing model from the issue text.