[Feature Request] Include Document XML in NuGet packages

[filzrev]

Is your feature request related to a problem? Please describe.
Intellisense is not works when using Microsoft.DocAsCode.App package.
Because NuGet package don't contains xml documentation files.

Describe the solution you'd like
Enable xml document generation by enabling following settings in `Directory.Build.Props.

<PropertyGroup>
  <GenerateDocumentationFile>true</GenerateDocumentationFile>
</PropertGroup>

Currently 4000+ warnings generated when this settings enabled.
It needs to solve these warnings.

• Change class visibility from public to internal if it's not intended used.
• Disable GenerateDocumentationFile for test projects.
• Add documentation comments.

Describe alternatives you've considered
Enable GenerateDocumentationFile only when creating NuGet packages .
And ignore warnings.