Restore original README content above Query capabilities overview

[Copilot]

Per feedback on #1696, the README introduction was simplified too aggressively. This PR restores the original content from main above the "Query capabilities overview" section while preserving the new query documentation.

Restored sections:

• Original title: # JSON-Server (with hyphen)
• CI badge and callout blocks (IMPORTANT, NOTE)
• ## Install and ## Usage sections with detailed db.json5 examples
• ## Sponsors ✨ and ## Sponsorware sections

Preserved sections:

• ## Query capabilities overview and all subsequent documentation from feat: add _where filtering, use new op separator, drop _start, _end, _limit #1696

This maintains the established project structure while introducing the new query filtering documentation.

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/typicode/json-server/contents/README.md
  • Triggering command: /usr/bin/curl curl -s -H Accept: application/vnd.github.v3.raw REDACTED (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)