Update model-dimensions.md

[Delanceyyy]

What this PR does

This PR replaces outdated references to the Nexis dataset in model-dimensions.md
with a new example dataset (example_data.csv), since Nexis data is no longer publicly available.

Fixes #7655

Why this change

• Nexis dataset is no longer accessible to the public.
• The updated example (example_data.csv) keeps the documentation functional and self-contained.
• No code or behavior changes — documentation only.

Test me

Before: https://docs.terria.io/guide/connecting-to-data/customizing-data-appearance/model-dimensions/
After: https://deploy-preview-7724--terriajs-docs-v8.netlify.app/guide/connecting-to-data/customizing-data-appearance/model-dimensions/

Checklist

• Documentation updated in doc/.
• Verified rendered Markdown preview looks correct.
• No code or functional changes required.

[CLAassistant]

All committers have signed the CLA.

[Delanceyyy]

Hi! This PR replaces the outdated Nexis dataset reference with a generic example dataset
(example_data.csv) as Nexis data is no longer public.
Documentation only, no code changes.
Thanks for reviewing!

[niktayv]

@Delanceyyy Thank you for your effort!
Clarifying how model dimensions work is much appreciated.

You can check the preview of the generated documentation here https://deploy-preview-7724--terriajs-docs-v8.netlify.app/guide/connecting-to-data/customizing-data-appearance/model-dimensions/
If you open the "example link", you will see the map but no data, which is expected because the example_data.csv file does not exist.

It looks like we can do better by using a real CSV file, let's say wwwroot/test/2011Census_AUST_STE.csv, so that the example link on the documentation page works and matches the screenshot.

Additionally, this example can be added to the demo catalogue wwwroot/init/simple.json with some helpful description.