📚 Doc: Add more validation examples (#3369)

* add examples on valudator guid

* ref: return prev validation comment

docs/guide/validation.md

@@ -8,8 +8,7 @@ sidebar_position: 5
 
 Fiber provides the [Bind](../api/bind.md#validation) function to validate and bind [request data](../api/bind.md#binders) to a struct.
 
-```go title="Example"
-
+```go title="Basic Example"
 import "github.com/go-playground/validator/v10"
 
 type structValidator struct {
@@ -42,3 +41,71 @@ app.Post("/", func(c fiber.Ctx) error {
     return c.JSON(user)
 })
 ```
+
+```go title="Advanced Validation Example"
+type User struct {
+    Name     string `json:"name" validate:"required,min=3,max=32"`
+    Email    string `json:"email" validate:"required,email"`
+    Age      int    `json:"age" validate:"gte=0,lte=100"`
+    Password string `json:"password" validate:"required,min=8"`
+    Website  string `json:"website" validate:"url"`
+}
+
+// Custom validation error messages
+type UserWithCustomMessages struct {
+    Name     string `json:"name" validate:"required,min=3,max=32" message:"Name is required and must be between 3 and 32 characters"`
+    Email    string `json:"email" validate:"required,email" message:"Valid email is required"`
+    Age      int    `json:"age" validate:"gte=0,lte=100" message:"Age must be between 0 and 100"`
+}
+
+app.Post("/user", func(c fiber.Ctx) error {
+    user := new(User)
+    
+    if err := c.Bind().Body(user); err != nil {
+        // Handle validation errors
+        if validationErrors, ok := err.(validator.ValidationErrors); ok {
+            for _, e := range validationErrors {
+                // e.Field() - field name
+                // e.Tag() - validation tag
+                // e.Value() - invalid value
+                // e.Param() - validation parameter
+                return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{
+                    "field": e.Field(),
+                    "error": e.Error(),
+                })
+            }
+        }
+        return err
+    }
+    
+    return c.JSON(user)
+})
+```
+
+```go title="Custom Validator Example"
+// Custom validator for password strength
+type PasswordValidator struct {
+    validate *validator.Validate
+}
+
+func (v *PasswordValidator) Validate(out any) error {
+    if err := v.validate.Struct(out); err != nil {
+        return err
+    }
+    
+    // Custom password validation logic
+    if user, ok := out.(*User); ok {
+        if len(user.Password) < 8 {
+            return errors.New("password must be at least 8 characters")
+        }
+        // Add more password validation rules here
+    }
+    
+    return nil
+}
+
+// Usage
+app := fiber.New(fiber.Config{
+    StructValidator: &PasswordValidator{validate: validator.New()},
+})
+```