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

[filzrev]

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

note doc comment tag is 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 <note> tags are defined as root element.
If it's defined inside of <summary> or <remarks> element. It works as expected.

Describe the solution you'd like

1. Add Notes property to XmlComment.cs class.
2. Store XSL transformed notes to Notes property.
3. Render notes HTML if exists.