add links to content of the API documentation page (#182)

(cherry picked from commit 4c3e5ad)

Author: Chris Cho

source/api-docs.txt

@@ -2,6 +2,9 @@
 API Documentation
 =================
 
+- `Spark Connector for Scala 2.13 <https://www.javadoc.io/doc/org.mongodb.spark/{+artifact-id-2-13+}/{+current-version+}/index.html>`__
+- `Spark Connector for Scala 2.12 <https://www.javadoc.io/doc/org.mongodb.spark/{+artifact-id-2-12+}/{+current-version+}/index.html>`__
+
 .. toctree::
    :titlesonly: