Docs: fix docs build task

In Directory.Build.targets added PackageDownload deirectives
for packages used by Fornax and a target that copies dlls from
these downloaded packages to packages/docs directory as these
are no longer managed by Paket. Package versions are exact
because PackageDownload requires them to be exact.

Directory.Build.targets

@@ -8,4 +8,24 @@
                           IsImplicitlyDefined="true"
                           Version="6.0.0" />
     </ItemGroup>
+    <!-- Download and copy packages used by Fornax as these are no longer managed using Paket -->
+    <ItemGroup>
+        <PackageDownload Include="FSharp.Formatting" version="[4.0.0]" />
+        <PackageDownload Include="Markdig" version="[0.34.0]" />
+        <PackageDownload Include="Newtonsoft.Json" version="[13.0.3]" />
+    </ItemGroup>
+    <Target Name="PostBuild" AfterTargets="PostBuildEvent">
+        <ItemGroup>
+            <FSharpFormattingDlls Include="$(NugetPackageRoot)fsharp.formatting/4.0.0/lib/netstandard2.0/*.dll"/>
+        </ItemGroup>
+        <Copy SourceFiles="@(FSharpFormattingDlls)"
+              DestinationFolder="$(MSBuildThisFileDirectory)packages/docs/FSharp.Formatting/lib/netstandard2.0/"
+              ContinueOnError="false" />
+        <Copy SourceFiles="$(NugetPackageRoot)markdig/0.34.0/lib/netstandard2.0/markdig.dll"
+              DestinationFolder="$(MSBuildThisFileDirectory)packages/docs/Markdig/lib/netstandard2.0/"
+              ContinueOnError="false" />
+        <Copy SourceFiles="$(NugetPackageRoot)newtonsoft.json/13.0.3/lib/netstandard2.0/Newtonsoft.Json.dll"
+              DestinationFolder="$(MSBuildThisFileDirectory)packages/docs/Newtonsoft.Json/lib/netstandard2.0/"
+              ContinueOnError="false" />
+    </Target>
 </Project>