Improvements to Training-ML-Agents (#3776)

* Improvements to Training-ML-Agents

- Removed duplicate documentation
- Moved CLI descriptions to learn.py
- Reorganized "Training with mlagents-learn" into 5 sub-sections

* fixed formatting errors and incorporated minor feedback

* minor improvement

* Minor formatting.

* fixed run-id references

* Keeping link to use Inference consistent with master

Will update the UIE page in a separate PR.

* Squashed commit of the following:

commit 9600d0f
Author: Marwan Mattar <marwan@unity3d.com>
Date:   Tue Apr 14 17:45:33 2020 -0700

    Various doc improvements (#3775)

    * Various doc improvements

    For Using-Virtual-Environment.md:
    - Made a note regarding updating setuptools and pip.
    - Changed lists from "-" to "*"

    For Using-Tensorboard.md:
    - Changed the ordered list to use "1."

    For Training-on-Microsoft-Azure-Custom-Instance.md:
    - Deleted as it was not linked anywhere

    For FAQ.md
    - Removed stale issues given upgrade to 2018.3

    For Readme.md
    - Added links for Reward Signals, Self-Play and Profiling Trainers

    For Learning-Environment-Executable.md
    - Changed the ordered list to use "1."

    For Learning-Environment-Examples.md
    - Minor rewording of intro paragraphs

    * consolidating custom instances page in main page

    So we have a single page for Azure.

    Adding warning note for deprecated docs

    * Fixing doc links that are failing CI

commit 34db9c2
Author: Vincent-Pierre BERGES <vincentpierre@unity3d.com>
Date:   Tue Apr 14 14:06:16 2020 -0700

    Removing dark mode from screenshots (#3772)

commit 1e164cc
Author: Chris Elion <chris.elion@unity3d.com>
Date:   Tue Apr 14 13:08:18 2020 -0700

    [MLA-488] Fix observations in demonstration drawer (#3771)

    * WIP observation shapes

    * WIP observation shapes

    * fix Observation shape serialization

    * rename field, redo screenshot

    * docstring

    * changelog

* Revert "Squashed commit of the following:"

This reverts commit 819e8d5.

* Removing debugging section.

* Prettier formatting

* Fixing Prettier oddity