Add Documentation attributes

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

Oqtane.Shared/Documentation/InternalApi.cs

@@ -0,0 +1,28 @@
+using System;
+
+namespace Oqtane.Documentation
+{
+    /// <summary>
+    /// This attribute serves as metadata for other things to mark them as internal APIs.
+    /// Use this on stuff you want to document publicly, but mark as internal so people are warned
+    /// </summary>
+    [AttributeUsage(AttributeTargets.All, AllowMultiple = true, Inherited = false)]
+    [PublicApi]
+    // ReSharper disable once InconsistentNaming
+    public class InternalApi_DoNotUse_MayChangeWithoutNotice: Attribute
+    {
+        /// <summary>
+        /// The `[InternalApi_DoNotUse_MayChangeWithoutNotice]` attribute can be used without additional comment. 
+        /// </summary>
+        // Important note - this constructor looks unnecessary, because comment is optional in the other constructor
+        // but we need it because of a minor issue in docfx
+        public InternalApi_DoNotUse_MayChangeWithoutNotice() { }
+
+        /// <summary>
+        /// Constructor with optional comment `[InternalApi_DoNotUse_MayChangeWithoutNotice(some-comment)]`. 
+        /// </summary>
+        /// <param name="comment">Reason why it's internal, optional</param>
+        public InternalApi_DoNotUse_MayChangeWithoutNotice(string comment) { }
+
+    }
+}