Survey ground-truth sources for evaluation

[katjabercic]

• Agda-Unimath: already imported. Curated, formal, but narrow in scope (univalent foundations).
• nLab: already imported. Check it's scope (i.e. do they have articles on people or books).
• ProofWiki: already imported. Check coverage.
• Others? e.g. EoM, Wolfram MathWorld cross-links, Wikidata "same as" properties, Lean's Mathlib4 docstrings linking to Wikipedia.