Document requirements for downloaded demonstrations

[sylvesterkaczmarek]

Summary

Improves the documentation around downloaded Stone Soup demonstrations, addressing the remaining usability problem described in #1050.

Sphinx-Gallery makes the generated notebook/source downloads convenient, but some demonstrations depend on repository data files, optional packages, generated assets, or external downloads that are not present in every generated archive. This can leave a locally downloaded demo looking self-contained when it is not.

The issue discussion already suggested adding guidance alongside the demonstration downloads so users know to check the full demo page and obtain the additional resources. This change adds that guidance to the demonstrations gallery introduction.

Change

The demonstrations landing page now:

• explains that generated downloads may require supporting data, assets, packages, or network downloads;
• links directly to the repository data used by AIS_Solent_Tracker, OpenSky_Demo, and UAV_tutorial;
• notes that the AIS and OpenSky demos use folium, which is available through the development extra;
• points Video_Processing users to the video and ultralytics extras;
• makes the network requirements of the video download and first-use model-weight download explicit;
• recommends a repository checkout, plus pre-fetching external assets, for restricted or isolated environments.

This complements #1051, which added the OpenSky CSV/icon links, by providing the gallery-level guidance suggested in the discussion on #1050.

Scope

Documentation only. The Sphinx-Gallery archive generation itself is unchanged.

Closes #1050