docs: add integrations section to README and ecosystem docs

[cwing-nvidia]

README

• add supported RL frameworks
• add environment library integrations
• update link from NeMo framework to ecosystem docs page. updating surrounding copy as needed

Ecosystem docs page
right now the docs page is nvidia is specific, this should be made more general; providing context how Gym relates to the rest of NeMo framework but also describe how Gym fits into the broader RL training ecosystem. then we can link to the supported RL frameworks (can we also mention the WIP items?) + provide a CTA to open a PR if there is one they'd like added or let us know, and also environment library integrations

[lbliii]

staging update exploration here #612