Add UbuCon presentation

[justinyoo]

Summary

Add a standalone UbuCon-styled Marp deck with event-specific opening and closing artwork, and publish it at /ubucon/ through the existing GitHub Pages workflow.

Change type

• Presentation content
• Presentation design
• PowerShell
• Bash
• Bicep
• Documentation or repository maintenance

Validation

• Marp HTML generation succeeds.
• Updated slides were visually reviewed.
• PowerShell files parse without errors.
• Bash files pass bash -n.
• Bicep files build successfully.
• No generated files, credentials, or sensitive data are included.

Additional checks:

• Verified Korean, English, and UbuCon decks build in the Pages directory structure.
• Verified the UbuCon opening and closing artwork resolves from the shared assets directory.
• Verified the opening and closing slide layouts in a browser.

Screenshots

Not included.

Related issues

None.