Add at least some mainpage for C++ docs.

docs/mainpage.dox

@@ -0,0 +1,25 @@
+// Copyright (c) Facebook, Inc. and its affiliates.
+// This source code is licensed under the MIT license found in the
+// LICENSE file in the root directory of this source tree.
+
+namespace esp {
+
+/** @mainpage Habitat Simulator
+
+A flexible, high-performance 3D simulator with configurable agents, multiple
+sensors, and generic 3D dataset handling (with built-in support for
+MatterPort3D, Gibson, Replica, and other datasets).
+
+When rendering a scene from the Matterport3D dataset, *Habitat-Sim* achieves
+several thousand frames per second (FPS) running single-threaded, and reaches
+over 10,000 FPS multi-process on a single GPU!
+
+@m_class{m-note m-warning}
+
+@par
+    Docs are currently very much a work-in-progress and are incomplete :(
+    * **But it's getting better!**
+
+*/
+
+}