feat: add GitHub link to NemoGym docs

[joeychuavertex]

The NemoGym documentation (https://docs.nvidia.com/nemo/gym/main) does not currently provide a clear link to the GitHub repository. This makes it harder for users to find the source code, report issues, or contribute. Users should be able to easily navigate from the NemoGym docs to the GitHub repository.

[cwing-nvidia]

@lbliii looks like this recently removed in #1413 - could this be added back?