Add public page for closed conferences

[Copilot]

Pull Request Template: Add a Conference or Workshop

Thank you for contributing! Most PRs are to add a new conference or workshop. The following are to help ensure you've added everything correctly:

Conference/Workshop Details

• Name, Location, Date(s), Website URL and Status are required
• X / Twitter is Optional
• Location should be City + Country or Online
• Status can include information about registration, pricing, calls for proposals and can include links to all the above.

Checklist

• I have added the conference/workshop to the correct YAML file (_data/current.yml or _data/past.yml)
• The entry includes name, location, date(s), url and accurate status
• The entry is in chronological order (soonest to furtherest away)
• If there are any special characters in the name field (: or ; or '), the name must be in quotes (")
• I have checked for duplicates to avoid listing the same event twice
• Build runs successfully

Additional context

This adds a new public /closed/ page to track conferences that have shut down or ended their annual run, including first and last known event dates. It also seeds the new page with the initial GreaTest Quality entry.

• Page + routing

  • Adds closed.html with a dedicated listing for closed conferences
  • Exposes the page in the main navigation as Closed

• Data model

  • Adds _data/closed.yml
  • Introduces first_date and last_date fields for closed-conference history
  • Seeds the first record with:
    • GreaTest Quality
    • Zurich, Switzerland
    • first event: June 2, 2023
    • last event: May 23, 2025

• Contributor guidance

  • Updates README.md to document closed.yml and the new closed-conference fields

• Rendered shape

- name: GreaTest Quality
  location: Zurich, Switzerland
  first_date: June 2, 2023
  last_date: May 23, 2025
  url: https://www.greatestquality.ch?utm_source=testingconferences
  status: Annual conference discontinued after 2025 and replaced by smaller community meetups.