mono-docs: Update DocFX version and use online symbol lookup

Version 2.45 of DocFX seems much much faster than previous 2.40, but could just
be a matter of stale caches. Anyway, no breakages have been detected.
Using an online symbol resolution server (https://xref.docs.microsoft.com)
adds some more links for .NET symbols (and no noticeable slowdown).

doc/docfx/docfx.json

@@ -16,6 +16,7 @@
     }
   ],
   "build": {
+    "xrefService": [ "https://xref.docs.microsoft.com/query?uid={uid}" ],
     "content": [
       {
         "files": [

doc/docfx/setup.sh

@@ -25,7 +25,7 @@ fi;
 #
 if [ ! -d "bin" ]; then
   rm -rf docfx.zip bin
-  wget https://github.com/dotnet/docfx/releases/download/v2.40.4/docfx.zip
+  wget https://github.com/dotnet/docfx/releases/download/v2.45/docfx.zip
   unzip -q docfx.zip -d bin
   rm docfx.zip
 else