[docs] Add doxygen to path in build

The documentation build was failing because it could not find the doxygen
executable.

Add doxygen path to `$env:PATH` in build script. Add some messages to debug
failures that might happen in future.

Fixes #1086

Co-authored-by: Christopher Warrington <chwarr@microsoft.com>

appveyor.yml

@@ -162,6 +162,10 @@
 
                 $env:Path = "$machinePath;$userPath"
 
+                $env:Path += "C:\Program Files\doxygen\bin"
+
+                Get-Command doxygen,pandoc
+
             }
 
             if (($env:BOND_BUILD -eq 'C++') -and (-not ($env:BOND_CMAKE_FLAGS -match '-DBOND_ENABLE_GRPC=FALSE'))) {