Docs: Replace vendor descriptions with neutral third-party links

[kevinjqliu]

Summary

Rename the Vendors page to Third-party ecosystem and replace long vendor descriptions with names and external links. This keeps third-party product claims off the Iceberg website and reduces the risk of implied endorsement.

The page also adds a non-endorsement disclaimer and a neutral listing policy. Future entries may include a brief neutral description, but the Iceberg project does not verify third-party claims.

Other ASF projects use similar ecosystem pages:

• Apache Airflow includes a prominent non-endorsement disclaimer and accepts additions through public channels.
• Apache Cassandra identifies entries as unsupported third-party offerings and accepts additions by pull request.

Scope and follow-ups

This PR intentionally reuses existing listing names and links to keep the change focused on the page structure and listing policy. A follow-up audit should review:

• Generic destinations: BladePipe, Collate, Stackable, and Teradata.
• Stale or cross-project destinations: CelerData/PhoenixAI, Upsolver/Qlik, and VeloDB/Apache Doris.

Validation

• Markdown lint
• Documentation builds on Linux and macOS

Preview

See https://github.com/kevinjqliu/iceberg/blob/kevinjqliu/codex-third-party-ecosystem/site/docs/vendors.md rendering

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AI Disclosure

• Model: GPT-5
• Platform/Tool: Codex
• Human Oversight: I reviewed everything 😄
• Prompt Summary: Refactor the vendor page into a neutral third-party ecosystem directory with ASF-aligned governance rules.