Add RepoCloud one-click deploy button to README

[cosark]

Summary

Adds a RepoCloud deploy button alongside the existing Heroku and DigitalOcean buttons in the "Deploying to a production server" section.

Changes

• Added RepoCloud button with height="20" to match existing Heroku/DigitalOcean button size
• Updated section text to mention RepoCloud alongside Heroku and DigitalOcean

Why

RepoCloud offers managed hosting for open-source applications. This gives users an additional one-click option to deploy their own Unleash instance.

Notes

• Used HTML <img> tag with explicit height="20" to ensure pixel-perfect alignment with the existing 20px-tall Heroku and DigitalOcean buttons
• The RepoCloud SVG natively renders at 42px, so the height constraint is required for visual consistency

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