Fix BuildDocs.ps1 -Publish error

This change fixes an issue with the BuildDocs.ps1 script that causes it
to throw an error when Remove-Item is called to delete the old
documentation repo folder.  It now uses Test-Path to check for the
folder's existence before attempting to delete it.

Author: daviwil

scripts/BuildDocs.ps1

@@ -36,7 +36,9 @@ else {
 
     if ($Publish.IsPresent) {
         # Delete the existing docs repo folder
-        Remove-Item -Path $docsRepoPath -Recurse -Force -ErrorAction Stop | Out-Null
+        if (Test-Path $docsRepoPath) {
+            Remove-Item -Path $docsRepoPath -Recurse -Force -ErrorAction Stop | Out-Null
+        }
 
         # Clone the documentation site branch of the Editor Services repo
         git clone -b gh-pages https://github.com/PowerShell/PowerShellEditorServices.git $docsRepoPath