Add "Creating Custom Robot Models for MuJoCo" to the wiki - Team C

[FD-Shubham]

The article covers the full pipeline from physical robot design to a working MuJoCo simulation, including URDF and MJCF model authoring, physics parameter tuning, and testing strategies.

Value to the Knowledgebase:
MuJoCo is increasingly the standard simulator for robotics research and RL, but the learning curve for model authoring is steep. This article consolidates practical knowledge that is otherwise scattered across the MuJoCo docs, ROS wikis, and community forums, giving contributors a single reference to go from a CAD design to a simulation-ready robot.