KAFKA-20737: Add CI guard for archived release links

[unknowntpo]

Problem

When a release moves from Supported releases to Archived Releases, its Apache release artifact links must move from the active download infrastructure to https://archive.apache.org/dist/kafka/.... This is currently a manual step and is easy to miss.

Approach

The new standard-library Python checker:

1. Reads only the ## Archived Releases section of content/en/community/downloads.md.
2. Extracts URLs outside Markdown code fences.
3. Fails when a Kafka distribution link still points to an active Apache endpoint:
   • downloads.apache.org/kafka/...
   • dlcdn.apache.org/kafka/...
   • Apache closer.lua redirector
4. Leaves unrelated links alone, including Docker Hub, Kafka blog/documentation, archive.apache.org, and the ASF KEYS link.

The check is static and makes no network requests, avoiding flaky external-link validation.

CI integration

• Pull requests into markdown: run the checker unit tests, then check the real downloads page.
• Deployment workflow: check only the real downloads page before building, so a direct push cannot deploy invalid archived links.
• Make targets provide the same commands locally.

Apache Jira: https://issues.apache.org/jira/browse/KAFKA-20737

Validation

• make check-archived-release-links-test (15 tests)
• make check-archived-release-links
• git diff --check origin/markdown...HEAD