add filter for events and archive tag

[mariopesch]

This pull request includes several updates to event files and the layout for displaying events. The key changes involve adding an "archive" field to events and updating the layout to filter out archived events.

Updates to event files:

• Added archive: true to multiple event files to mark them as archived, including content/events/2024-10-12-codeweek_kickoff.md, content/events/2024-10-14-iCODE_MS.md, content/events/2024-10-22-jugendhackt.md, content/events/2024-10-24-codeweek_fashion.md, and content/events/2024-11-13-hackathon.md. [1] [2] [3] [4] [5]

• Added a new event file content/events/2025-04-01-hyc-hackathon.md with details about a hackathon for teachers.

Updates to layout:

• Modified layouts/_default/events.html to include logic for filtering out archived events and sorting events by date.

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