[Feature Request] Include Document XML in NuGet packages

**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.

```

true

```

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.