Add Documentation attributes

part of https://github.com/oqtane/oqtane.framework/issues/570

Oqtane.Shared/Documentation/PrivateApi.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace Oqtane.Documentation
+{
+    /// <summary>
+    /// This attribute marks classes, methods, etc. as private APIs
+    /// So they should _not_ be publicly documented.
+    /// By default, all APIs are private, so you only need this attribute on children of classes marked with `[PublicApi]`.
+    /// </summary>
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+    [PublicApi]
+    public class PrivateApi: Attribute
+    {
+        /// <summary>
+        /// The `[PrivateApi]` attribute can be used without additional comment. 
+        /// </summary>
+        // Important note - this constructor looks unnecessary, because comment could be optional in the other constructor
+        // but we need it because of a minor issue in docfx
+        public PrivateApi() { }
+
+        /// <summary>
+        /// Constructor with optional comment `[PrivateApi(some-comment)]`. 
+        /// </summary>
+        /// <param name="comment">Reason why it's private, optional</param>
+        public PrivateApi(string comment) { }
+    }
+}