feat: add Artemis II mission artifact page and refine sidebar styles

This commit adds a complete NASA Artemis II mission documentation page, including:
1. 6 new content blocks for mission sections: crew manifest, system ignition, mission route, lunar flyby, return/future, and sources
2. Shared source link component and data files for static content and assets
3. Updated sidebar CSS to clean up icon link styling and remove redundant styles

Also removed the deprecated `.canvas-sidebar-footer-icon-link` selector and consolidated button hover states.

Author: Sayhi-bzb