Add OpenUSD MuJoCo setup scripts and wrapper examples

[dpeyton8]

This adds the initial OpenUSD/MuJoCo setup files for testing USD asset imports.

Included:

• Runtime helper script for launching MuJoCo with USD plugin paths
• OpenUSD setup notes
• MJCF XML wrapper examples for loading USD assets

Scope:

• This is for USD asset/environment import testing.
• This does not include build artifacts, downloaded assets, or training code.
• Next step is to test Seahorse/G1 warehouse assets through the OpenUSD -> MuJoCo wrapper pipeline.