[NRRPLT-7825] Adapted the Ant from PyBullet to a SDF model.

* [NRRPLT-7825] Adapted the Ant from PyBullet to a SDF model.

* [NRRPLT-7825] Removed MuJoCo file, uncommented plugin from model until it is in the NRP.

* [NRRPLT-7825] Added the Ant to the Robots category.

Approved-by: Manos Angelidis <angelidis@fortiss.org>