[RLlib; Docs] Updated RLlib training example page

[worldveil]

This updates the RLlib training docs page to stop users from running into errors when running examples.

Added some pip install ... to help. Still running into this error, though:

% rllib train --env=PongDeterministic-v4 --run=A2C --config '{"num_workers": 8}'
Traceback (most recent call last):
  File "/Users/will/opt/anaconda3/envs/ray-docs/bin/rllib", line 5, in <module>
    from ray.rllib.scripts import cli
  File "/Users/will/opt/anaconda3/envs/ray-docs/lib/python3.7/site-packages/ray/rllib/__init__.py", line 5, in <module>
    from ray.rllib.env.base_env import BaseEnv
  File "/Users/will/opt/anaconda3/envs/ray-docs/lib/python3.7/site-packages/ray/rllib/env/__init__.py", line 1, in <module>
    from ray.rllib.env.base_env import BaseEnv
  File "/Users/will/opt/anaconda3/envs/ray-docs/lib/python3.7/site-packages/ray/rllib/env/base_env.py", line 3, in <module>
    from ray.rllib.env.external_env import ExternalEnv
  File "/Users/will/opt/anaconda3/envs/ray-docs/lib/python3.7/site-packages/ray/rllib/env/external_env.py", line 7, in <module>
    from ray.rllib.utils.annotations import PublicAPI
  File "/Users/will/opt/anaconda3/envs/ray-docs/lib/python3.7/site-packages/ray/rllib/utils/__init__.py", line 10, in <module>
    from ray.rllib.utils.numpy import sigmoid, softmax, relu, one_hot, fc, lstm, \
  File "/Users/will/opt/anaconda3/envs/ray-docs/lib/python3.7/site-packages/ray/rllib/utils/numpy.py", line 2, in <module>
    import tree
ModuleNotFoundError: No module named 'tree'

[sven1977]

Doesn't gym come with pip install ray[rllib]?

[sven1977]

Looks good to me. Thanks for the fixes. Just one question on the additional gym installation.

[worldveil]

OK, removed. Let's merge?