Add external URL link checking rule

[jeduden]

Context

gomarklint's external-link rule validates external URLs by making HTTP requests. mdsmith's MDS027 only checks cross-file references (local paths and heading anchors), not external URLs.

Proposed rule

• ID: TBD (next available MDS ID)
• Name: external-link-check
• What it detects: External URLs that return errors (404, timeouts, DNS failures)
• Severity: warning
• Autofix: no
• Default: off (requires network, slow)

Considerations

• Should be off by default (network dependency, latency)
• Support skip patterns (regex) for known-flaky URLs
• Caching to avoid redundant requests across files
• Timeout configuration
• Parallel requests with rate limiting

Discovered via comparison with gomarklint.