Docs: Describe use of MSMT with single-shell data

[Lestropie]

Quickly wrote a proposal change based on recent forum thread.

Only partially addresses #2265; should consider whether to:

• Combine CSD and MSMT CSD pages into a single page;
• Whether to add example commands or exemplar image output screenshots.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[Lestropie]

Superseded by #3198.