Update ExcelDnaDoc NuGet Package to support the new PackageReference format

Similar to Excel-DNA/ExcelDna#188 and Excel-DNA/ExcelDna#193, ExcelDnaDoc relies on PowerShell scripts to install/uninstall which are not supported in projects using PackageReference.

ExcelDnaDoc's `install.ps1` basically does two things:

1. Replicates everything that [ExcelDna.AddIn's `install.ps1`](https://github.com/Excel-DNA/ExcelDna/issues/193) does, and adds a reference to `ExcelDna.Documentation.dll` in the .dna file for it to be packed.

```xml

```

2. Adds a post-build event to run ExcelDnaDoc.exe and generate the CHM

For (1), I suggest we remove all the PowerShell scripts, and [document](https://docs.excel-dna.net) that when using ExcelDnaDoc's attributes, the developer needs to manually add the `Reference` entry to the `.dna` file.

For (2), I suggest we bundle an MSBuild script into the NuGet package, and make it part of the build as described in Excel-DNA/ExcelDna#32