docs: add documentation improvements

[devfeel]

Summary

This PR improves project documentation to address issue #233.

Changes

1. README.md Updates

• Added Group SetNotFoundHandle usage example
• Included behavior explanation for group-level 404 handling

2. CHANGELOG.md (New)

• Created changelog following Keep a Changelog format
• Documented new features from recent PRs:
  • Group SetNotFoundHandle (希望组路由支持SetNotFoundHandle #229)
  • Trailing Slash Redirect Configuration (url with trailingslah /hello/xxx/ match route /hello/:name should return 404 not 301 #245)
  • CORS Preflight Support (在路由组上注册跨域中间件时，复杂跨域请求会报CORS策略阻止错误。 #250)
  • Security vulnerability fixes

3. Go Doc Improvements

• Added comprehensive documentation comments to Group interface
• Documented all methods with their purpose and usage
• Added documentation for xGroup struct fields

Test Plan

• All existing tests pass
• Go Doc comments follow standard format
• CHANGELOG follows Keep a Changelog format

Screenshots

Group SetNotFoundHandle Example

// Create API group
apiGroup := server.Group("/api")

// Set group-level 404 handler
apiGroup.SetNotFoundHandle(func(ctx dotweb.Context) error {
    ctx.Response().Header().Set("Content-Type", "application/json")
    return ctx.WriteString(`{"code": 404, "message": "API resource not found"}`)
})

Resolves: #233

[devfeel]

Closing this PR as requested. Will resubmit after confirmation.