Align ExcelDnaDoc build with new ExcelDna.AddIn build

The ExcelDnaDoc package (http://mndrake.github.io/ExcelDnaDoc/index.html and https://github.com/mndrake/ExcelDnaDoc) provide automatic help file generation for Excel-DNA add-ins. It also installs some post-build events (and assumes that these run before the ExcelDna.AddIn build events).

We need to check how it is impacted by the new ExcelDna.AddIn build targets approach, and whether the package needs to be updated to align the build steps from ExcelDnaDoc with ExcelDna.AddIn.

@mndrake might be able to able to help, otherwise it might be suggested as a pull request to the ExcelDnaDoc project.