Tracker internal doc - wiki/tutorial (#626)

Summary:
Pull Request resolved: #626

Guide type doc to get started with TorchX Tracker API using
- Manifold
- Mast

Reviewed By: d4l3k

Differential Revision: D40516224

fbshipit-source-id: c6df675bf1703ed3a1ceb7a47b31fd05af14d77e

Author: kurman

docs/source/index.rst

@@ -59,6 +59,7 @@ Documentation
       fb/workspace.md
       fb/cogwheel.rst
       fb/named_resources.rst
+      fb/tracker.md
 
 
 Works With