Add docs output directory to .gitignore

docs/open-xml-docs is where the docfx build ends up - it appears to have out of date assets that are overwritten with the build. This change removes the directory and adds it to .gitignore

As part of this change, a default template is added to docfx.json so that local builds can work.

Author: twsouthwick

.gitignore

@@ -286,3 +286,5 @@ __pycache__/
 *.btm.cs
 *.odx.cs
 *.xsd.cs
+
+docs/open-xml-docs/

docs/docfx.json

@@ -48,7 +48,10 @@
         "**/*.yml": ["csharp", "vb"]
       }
     },
-    "template": [],
+    "template": [
+      "default",
+      "modern"
+    ],
     "dest": "open-xml-docs",
     "markdownEngineName": "markdig"
 }