Add Documentation attributes

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

Oqtane.Shared/Documentation/PublicApi.cs

@@ -0,0 +1,27 @@
+using System;
+
+namespace Oqtane.Documentation
+{
+    /// <summary>
+    /// This attribute marks classes, properties etc. as public APIs. 
+    /// Any API / code with this attribute will be published in the docs.
+    /// You can apply it to anything, but usually you will only need it on classes. 
+    /// </summary>
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+    [PublicApi]
+    public class PublicApi: Attribute
+    {
+        /// <summary>
+        /// The `[PublicApi]` attribute can usually 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 PublicApi() { }
+
+        /// <summary>
+        /// Constructor with optional comment `[PublicApi(some-comment)]`
+        /// </summary>
+        /// <param name="comment">Reason why it's public, optional</param>
+        public PublicApi(string comment) { }
+    }
+}