[Feature Request] Add support for `note` doc comment tags that is defined at root element.

**Is your feature request related to a problem? Please describe.**

`note` doc comment tag is [User-defined tags](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/xmldoc/recommended-tags#user-defined-tags) that is used by SandCastle.
And `note` tag seems to be supported by docfx by default. (`https://github.com/dotnet/docfx/blob/efc1c3df765a3514f2bc5c786e5d05c003a466fd/src/Docfx.Dotnet/Resources/XmlCommentTransform.xsl#L68-L94`)

But it's not works when `` tags are defined as root element.
If it's defined inside of `` or `` element. It works as expected.

**Describe the solution you'd like**
1. Add `Notes` property to [XmlComment.cs](https://github.com/dotnet/docfx/blob/efc1c3df765a3514f2bc5c786e5d05c003a466fd/src/Docfx.Dotnet/Parsers/XmlComment.cs) class.
2. Store XSL transformed notes to `Notes` property.
3. Render notes HTML if exists.