docs: add product persona docs for auditors, cooperatives, and meter operators

[jessicanath]

Summary

Closes #531

Adds docs/PERSONAS.md documenting the three SolarProof target user personas and their primary workflows.

Acceptance criteria

• Personas for auditors, cooperatives, and meter operators are defined
• Key user stories are listed for each persona (A1–A4, C1–C5, M1–M5)
• Docs include a prioritised feature table to inform future feature work

What's in the doc

Section	Contents
Auditor	Goals, 4 user stories, feature implications (public verifier, CSV export, retirement visibility)
Cooperative	Goals, 5 user stories, feature implications (dashboard, meter registration, governance UI)
Meter Operator	Goals, 5 user stories, feature implications (send-reading CLI, tracer-sim error links)
Prioritisation table	P0–P3 ranked features cross-referenced to persona

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