docs(contrib): add external industrial dataset guide (proposal)

[ulises-jeremias]

Description

This PR adds a contributor proposal for issue #342: a curated external industrial dataset guide to help external contributors discover public sources and map them to existing AssetOpsBench domains.

As a non-maintainer, I intentionally kept this scoped to documentation only.

Type of Change

• Documentation / Tutorial update

Related Issues

• Refs: Links to datasets #342

Changes

• Added docs/external-industrial-datasets.md:
  • starter dataset references,
  • mapping checklist to AssetOpsBench domains,
  • privacy/safety and provenance guardrails,
  • suggested ingestion workflow for contributor PRs.
• Added a small pointer in README.md under "Call for Scenario Contribution".

Why this scope

• No benchmark logic, scoring, or runner behavior is modified.
• Keeps the contribution low-risk while providing practical guidance for external contributors.

Checklist

• I have signed off my commits (DCO).