Redesign index.html: interactive-first landing page

The newsletter now lives in Outlook Newsletters. This page is the
'double-click' — where readers explore patterns interactively.

Changes:
- Hero section leads with the value prop (explore patterns, not archive)
- Pattern cards are the main content: title, builder, description,
  tech tags, and direct link to interactive walkthrough
- Newsletter callout directs to Outlook Newsletters for subscription
- Coming Soon section teases #5-#6
- Removed old issue archive list (lives in Outlook back-catalog)
- Nav bar includes all 4 interactive issues with explicit index.html paths
- Wider layout (800px vs 720px) for card grid

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: justinpreston