Update documentation for error-returning handlers and new serialization

[FumingPower3925]

Breaking change in celeris core

The HandlerFunc type has changed from func(*Context) to func(*Context) error in v1.0.0.

Documentation updates needed

All code examples must use the new signature:

Before:

s.GET("/hello", func(c *celeris.Context) {
    c.String(200, "Hello, World!")
})

After:

s.GET("/hello", func(c *celeris.Context) error {
    return c.String(200, "Hello, World!")
})

New features to document

1. Error-returning handlers — handlers return error, c.Next() returns error
2. HTTPError type — celeris.NewHTTPError(code, message) for structured errors
3. Error handling in middleware — err := c.Next() to intercept downstream errors
4. Unhandled error safety net — *HTTPError → writes Code+Message; bare error → 500
5. XML serialization — c.XML(code, v) response method
6. Content-type-aware binding — c.Bind(v) auto-detects JSON/XML from Content-Type
7. Explicit binding — c.BindJSON(), c.BindXML()
8. Content negotiation — c.Negotiate(), c.Respond() for auto-format selection
9. Response capture — c.CaptureResponse(), c.ResponseBody(), c.ResponseContentType()
10. Route-level middleware — s.GET("/path", handler).Use(middleware)

Note: ProtoBuf is NOT in core — available via goceleris/middlewares.

Reference

The core doc.go and README.md have already been updated with the new patterns.

[FumingPower3925]

Scope expanded: also cover content negotiation, response capture, route-level middleware.