Update ExcelDnaDoc NuGet Package to support the new PackageReference format

[augustoproiete]

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 does, and adds a reference to ExcelDna.Documentation.dll in the .dna file for it to be packed.

<Reference Path="ExcelDna.Documentation.dll" Pack="true" />

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 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