[chore] [editoral] Add info on selecting signal type

[thompson-tomo]

Changes

This adds in some guidance on selecting a signal type based on questions regarding what you want to do.

A slight re-ordering has been done to improve grouping/readability.

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Merge requirement checklist

• [x] CONTRIBUTING.md guidelines followed.
• [ ] Change log entry added, according to the guidelines in When to add a changelog entry.
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• [ ] Links to the prototypes or existing instrumentations (when adding or changing conventions)