docs: add Sourcey-generated API reference documentation

[patrick6x6]

Summary

Adds comprehensive API reference documentation generated by Sourcey from the echo Go module's godoc data.

What's included

• Full API reference for all exported types, functions, and interfaces across echo, echotest, and middleware packages
• Search functionality with full-text search across all API surfaces
• Structured navigation with package-level organization
• GitHub Actions workflow for automatic deployment to GitHub Pages

How it works

The docs are generated from the actual Go source code using sourcey godoc, which snapshots the godoc data and renders it as a navigable documentation site. This ensures the docs stay in sync with the code.

Deployment

The included GitHub Actions workflow (deploy-docs.yml) will automatically deploy the docs to GitHub Pages when changes are made to the docs/ directory on the main branch. Once merged, the docs will be live at labstack.github.io/echo.

Preview

The docs can be previewed locally by opening docs/api-reference/index.html in a browser.

Benefits

• Always up-to-date with the latest code
• Searchable API reference
• No manual documentation maintenance required
• Complements the existing documentation at echo.labstack.com