added validation section to docs

docs/validation.md

@@ -2,21 +2,57 @@
 
 QuickForm has two types of validation
 
-- Microsoft's `DataAnnotationsValidator`
+- `DataAnnotationsValidator`
 - `Blazored.FluentValidationValidator`
 
-## Before you start
-
-Make sure that:
-
-- QuickForm package is added to your project.
-- QuickForm is properly imported.
-- You have your fluent validation properly set up.
-
-## DataAnnotation Validation
-
-There are many data annotation attributes supported for validation,
-most built-into C#, but some specific to QuickForm
-
-Please refer to the [Customizing with attributes](attribute-customization.md) section for more 
-information about supported attributes
+## DataAnnotationAttributes
+
+```csharp
+public class User
+{
+    [Required]
+    public string Name { get; set; }
+
+    [Range(18, 100. ErrorMessage = "You must be 18 years or older to register")]
+    public int Age { get; set; }
+
+    [RegularExpression(@"^(?=.*[a-z])(?=.*[A-Z]).{8,32}$", ErrorMessage = "Password must have at least 8 characters, one uppercase letter, one lowercase letter...")]
+    public string Password { get; set; }
+}
+```
+
+## FluentValidation
+
+```csharp
+public class ProductCreate
+{
+    public string Name { get; set; }
+
+    public decimal Price { get; set; }
+}
+
+public class ProductCreateValidator : AbstractValidator<ProductCreate>
+{
+    public ProductCreateValidator(IProductRepository productRepository)
+    {
+        RuleFor(x => x.Name)
+            .NotEmpty()
+            .WithMessage("Name is required")
+            .MustAsync(async (name, product, cancellationToken) => {
+                var product = await productRepository.GetByName(name);
+                return product == null;
+            })
+            .WithMessage("Product with this name already exists");
+
+        RuleFor(x => x.Price)
+            .GreaterThan(0)
+            .WithMessage("Price must be greater than 0");
+    }
+}
+```
+
+
+!!! info
+
+    These validation rules will be automatically applied to your form, 
+    and all fields will get validated when they are changed, even asynchronously.

mkdocs.yml

@@ -14,6 +14,7 @@ nav:
   - Customization: customization.md
   - Validation: validation.md
   - Attributes: attributes.md
+  - API: api.md
 
 theme:
   name: material