add csharp docs latest to extlinks (#567)

Author: Chris Cho

conf.py

@@ -69,6 +69,7 @@
     'mms-home': ('https://cloud.mongodb.com%s', ''),
     'guides': ('https://docs.mongodb.com/guides%s', ''),
     'java-docs-latest': ('https://mongodb.github.io/mongo-java-driver/3.11/%s', ''),
+    'csharp-docs-latest': ('http://mongodb.github.io/mongo-csharp-driver/2.9%s', ''),
     'aws-docs': ('https://docs.aws.amazon.com/%s', ''),
     'wikipedia': ('https://en.wikipedia.org/wiki/%s', ''),
 }

source/drivers/csharp.txt

@@ -20,13 +20,13 @@ Introduction
 The official MongoDB C#/.NET Driver provides asynchronous interaction
 with MongoDB.
 
-- `Getting Started <http://mongodb.github.io/mongo-csharp-driver/2.8/getting_started/>`__
+- :csharp-docs-latest:`Getting Started </getting_started/>`
 
 - `Usage Guide <http://mongodb.github.io/mongo-csharp-driver/?jmp=docs>`__
 
 - `API Reference <http://api.mongodb.com/csharp/current?jmp=docs>`_
 
-- `Changelog <http://mongodb.github.io/mongo-csharp-driver/2.8/what_is_new/>`__
+- :csharp-docs-latest:`Changelog </what_is_new/>`
 
 - `Source Code <https://github.com/mongodb/mongo-csharp-driver>`__
 
@@ -36,8 +36,8 @@ Installation
 
 ``NuGet`` is the simplest way to get the driver. `MongoDB.Driver <http://www.nuget.org/packages/mongodb.driver>`__ should be used for all new projects.
 
-For more information, see `Installation
-<http://mongodb.github.io/mongo-csharp-driver/2.8/getting_started/installation/>`_.
+For more information, see :csharp-docs-latest:`Installation
+</getting_started/installation/>`.
 
 
 Connect to MongoDB Atlas
@@ -56,7 +56,7 @@ To connect to a `MongoDB Atlas <https://docs.atlas.mongodb.com/>`_ cluster, use
    var database = client.GetDatabase("test");
 
 
-See `Connecting <http://mongodb.github.io/mongo-csharp-driver/2.7/reference/driver/connecting/>`__
+See :csharp-docs-latest:`Connecting </reference/driver/connecting/>`
 for more information.