Clarify reference value, discovery, and Copilot setup

[chuks]

Summary

Makes the live Copilot reference discoverable and immediately actionable.

• adds a prominent integrations section and visual to the repository README
• turns the references index into an available-reference and audience guide
• answers why developers and enterprises need receiver-verifiable authority
• distinguishes GitHub access controls from complementary Ratify authority
• documents the exact public Copilot install and receiver setup path
• adds an open-source versus Ratify Verify decision guide and design-partner CTA
• removes stale pre-merge language

Verification

• direct install from the public repository succeeded with Copilot CLI 1.0.80
• ratify-authority appeared in the installed plugin list
• relative documentation targets verified
• Mermaid source and Markdown structure reviewed
• git diff --check