[#1490] add new mujoco example showing how to add gravity bodies

Example uses both Spice and planetEphemeris message states.

Author: schaubh

examples/_default.rst

@@ -217,6 +217,7 @@ It's recommended to study the first 6 scenarios in order:
   Chained Hinged Panel Deployment <mujoco/scenarioDeployPanels>
   Solar Radiation Pressure Model on Arbitrary Surfaces <mujoco/scenarioSRPInPanels>
   Landing on Asteroid with Contact Physics <mujoco/scenarioAsteroidLanding>
+  Earth-Moon Gravity with SPICE or PlanetEphemeris <mujoco/scenarioMJEarthMoonGravity>
   Docking between Two CubeSats <mujoco/scenarioSimpleDocking>
   Branching Panel Deployment with Locking Mechanisms <mujoco/scenarioBranchingPanels>
   Pointing with Reaction Wheels in MuJoCo <mujoco/scenarioAttitudeFeedbackRWMuJoCo>