Pin RL and Gym versions to avoid breaking changes

[shashank3959]

Describe the bug

The documentation / tutorial does not pin a specific commit of RL and Gym. Main branches for both are fast moving and introduce breaking changes often.

Simple things like naming changes can be breaking.

It may get auto-resolved with NeMo Gym support in the NeMo RL container, however

• We do not have Gym in the RL container right now.
• There are some users who may choose to not use containers.

Steps/Code to reproduce bug

Follow the steps in the docs

Expected behavior

NeMo RL and Gym interoperate cleanly, always. We guide users to use only compatible and tested versions.