[feature] : Document where to obtain lab U-Nets

[SylviaWhittle]

Is your feature request related to a problem?

In grain_finding.md we mention that one can optionally use U-Nets with TopoStats however we do not yet provide any U-Nets.

We should list the U-Nets that we make available through our publications. How we should do that can be discussed and implemented.

Describe the solution you would like.

Write up documentation on where to obtain our publicly available U-Nets that the lab has trained in the grain_finding.md documentation.

Describe the alternatives you have considered.

N/A

Additional context

N/A

[ns-rse]

Some options...

• Make them part of the website and available to download (would sit in docs/assets and we should include SHA256 checksums for the files so people can check the veracity of what they download).
• Bundle them into TopoStats itself is also an option but this may bloat the package size and impact on its hosting on PyPI.
• Deposit models in Zenodo or ORDA and link to them (makes the "dataset"/model citeable with a unique DOI).