chore: update dependencies and refactor README with architecture diagram

[gjchentw]

📝 Description

This PR updates the project dependencies to their latest versions and significantly improves the documentation to better explain the project's purpose and usage.

🛠️ Changes

📦 Dependencies

• Bumped Go version to 1.24.0.
• Updated github.com/gin-gonic/gin to v1.11.0.
• Ran go mod tidy to ensure go.sum is up to date.

📚 Documentation (README.md)

• Added Badges: Included status badges for CI, Go version, Go Reference, and License.
• Clarified Goals: Updated the introduction to clearly state the "Write Once, Reuse Everywhere" philosophy.
• Architecture Diagram: Added a Mermaid graph to visualize the relationship between Gin, Ginmill, and Feature packages (including a "ginmill" nod).
• Improved Examples:
  • Added a basic "Cheers" example.
  • Refined the "Mastodon API" example to show advanced usage.

✅ Verification

• Ran go test ./... - All tests passed.
• Ran govulncheck ./... - No vulnerabilities found.