docs: add RepoCloud as a deployment option

[cosark]

Adds RepoCloud as a one-click deployment option alongside the existing providers (DigitalOcean, Render, Railway, Zeabur).

RepoCloud provides managed cloud hosting for open-source applications with one-click deployment.

• Adds deploy button to README.md
• Links to: https://repocloud.io/details/Hi.Events/

[github-actions]

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