Ufactory Lite6 - MjSpec gripper attachment

[eufrizz]

Follow on from https://github.com/google-deepmind/mujoco_menagerie/pull/89#issuecomment-2323110997 Instead of separate models with different grippers, provide body and gripper model separately and attach them with the new MjSpec API. Changes:

• Add frame to attachment site in lite6.xml
• Add gripper narrow and wide as separate .xml
• Remove .xmls with body and gripper already attached
• Add example basic_attach.cc to demonstrate attaching with the MjSpec API (based on the mujoco basic.cc example)
• Add example code to README TODO:
• Add Python example when MjSpec API works