docs(readme): evolve supporter section into Sponsors section (sub2api-style)

[diegosouzapw]

Summary

Evolves the Official Supporters section into a full Sponsors section, taking the best of the two reference formats (Wei-Shaw/sub2api and router-for-me/CLIProxyAPI) and adding what neither has:

• "Want to appear here?" call-to-action with the maintainer contact and the audience pitch
• Kimi entry in the sub2api thank-you format, selling the sponsor's product (K3: 1M context, native vision, works with Claude Code/Codex)
• "Where the sponsorship goes:" explicit transparency label — release pipeline (merge validation powered by Kimi K3) + feature development
• Shields badge in the official Kimi blue #1783FF, matching the dashboard provider-card accent shipped in feat(dashboard): pin Kimi providers first in category + official supporter card accent #7775
• "Get a Moonshot API key →" CTA (plain link for now; becomes the disclosed partner tracking link when Kimi provides it)

Docs-only change on top of the section merged in #7770.

Refs: Kimi partnership pilot.

[gemini-code-assist]

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