Add BXI Elf2 humanoid robot description

[nnoott]

This PR adds the BXI Elf2 humanoid robot description to the repository. The model originates from bxi robot_models repository. It is an official release, it is a realistic description used to develope dynamic algorithms in both simulation and hardware.

The Elf Robot itself is under rapid developement progress. I believe it deserves a place in the Awesome Robot Descriptions repository as it is an awesome one.

BXI Elf2 info:

Name	Maker	Formats	License	Visuals	Inertias	Collisions
Elf2	BXI Robotics	MJCF, URDF	Apache-2.0	✔️	✔️	✔️

Elf2 robot picture added to the end of Gallery:
img src="gallery/elf2.png" alt="Med 14" width=100

[stephane-caron]

LGTM, thank you for this addition 👍

[stephane-caron]

Following up I have also linked the URDF description from robot_descriptions.py in robot-descriptions/robot_descriptions.py#131 😋

What you prefer for the MJCF description? Shall it load the one from https://github.com/bxirobotics/robot_models or do you prefer to wait for google-deepmind/mujoco_menagerie#158 to be merged, and maintain a single official MJCF in MuJoCo Menagerie?

[nnoott]

Following up I have also linked the URDF description from robot_descriptions.py in robot-descriptions/robot_descriptions.py#131 😋

What you prefer for the MJCF description? Shall it load the one from https://github.com/bxirobotics/robot_models or do you prefer to wait for google-deepmind/mujoco_menagerie#158 to be merged, and maintain a single official MJCF in MuJoCo Menagerie?

Thanks Stephane! Please use our repo: https://github.com/bxirobotics/robot_models . We would update it more often, like adding more robots.
We are still waiting for google-deepmind/mujoco_menagerie#158 to go through.

[stephane-caron]

Sounds good, the MJCF description was added in robot-descriptions/robot_descriptions.py#134 👌