Clearly Mark Different Sets of Atari Games In Documentation

The environments previously in Gym, the Atari 56 environments from DeepMind, and so on should be popped out in their own doc categorizations for researchers and students. Like some sort of page listed on the sidebar with links to the various notable particular sets of Atari environments

Start by locating the existing documentation sidebar and Atari environment pages, then review how current environment groups are organized. Done means notable Atari sets, including the environments previously in Gym and DeepMind's Atari 56, have clear sidebar entries and links to their respective documentation.

Written by the indexing model from the issue text.