Add licensing overview page

[jasnoyaeger]

Summary

• Adds a new Licensing Overview page to the product guide covering System IDs, license types, reinstallation, and reactivation
• Adds nav entry under System Settings, before the existing "Licensing and Updates" page
• Adds cross-link from existing Licensing and Updates page to the new overview
• Aligns support team naming ("VergeIO team") across licensing pages

Closes #389

Test plan

• Verify mkdocs serve renders the new page correctly
• Confirm nav placement under System Settings > Licensing Overview
• Review cross-links to existing licensing and air-gap docs

🤖 Generated with Claude Code

[sethesterline]

tested offline and looks good. verified against 26.1