Fix documentation for "other helpful functions" in eta_x docs

The documentation of "Other helpful functions" in the ETA-X -> Common functions section is currently empty. It should contain documentation for the following functions and classes:

• eta_x.common.is_vectorized_env
• eta_x.common.is_env_closed
• eta_x.common.episode_results_path
• eta_x.common.episode_name_string