docs: add custom domain screenshots and UI walkthrough

[sebastianiv21]

Summary

• Add product screenshots to the custom domain and publishing guides, walking users through the real in-app flow (publish panel, connect vs buy, Entri DNS, domain status).
• Expand connect-a-custom-domain.md with buy-through-Kite steps, subdomain checkbox, upgrade wording, and disconnect instructions merged with existing Entri one-click/manual content.
• Move the first-publish Site Domain walkthrough to your-kite-url.md and link from how-to-publish.md to avoid duplication.
• Add CSS so doc screenshots stay sharp at native size, centered in the content column, with light/dark theme framing.

Test plan

• Run cd website && npm run build and confirm it passes
• Preview /publishing/how-to-publish, /publishing/your-kite-url, and /custom-domains/connect-a-custom-domain
• Verify screenshots are centered and not upscaled/pixelated
• Confirm buy-domain and connect-existing paths both read clearly for non-technical users

Made with Cursor

[salevine]

Good