Enable GitHub Discussions on main repo

[zeevdr]

Description

Enable GitHub Discussions on the main decree repo as the central place for Q&A, ideas, and general conversation. Keeps the Issues tab clean for actionable bugs and tasks.

Acceptance criteria

• Enable Discussions on decree repo (Settings → Features → Discussions)
• Set up categories: Q&A, Ideas, Show & Tell, General
• Add a pinned welcome discussion explaining what goes where (Issues vs Discussions)
• Link to Discussions from README and SUPPORT.md
• Other repos (SDKs, UI) should NOT enable Discussions — point to main repo instead

Notes

• Centralized discussions prevent fragmentation across repos
• Q&A category supports "mark as answer" which helps future users