Update self-host docs for cleaner onboarding

[Uarmagan]

Summary

• Remove the inline self-host installer command from the main README and the self-host runtime README
• Reword the top-level self-host overview to sound plainer and more direct
• Expand the server guide with clearer DNS, firewall, Caddy, and public-service guidance

Testing

• git diff --check
• bun run lint (completed; existing repo warnings remain unrelated to these docs changes)