simplify where install directions are

[charnpreetsingh]

• Removed details concerning html_sidebars since we account for that case by automatically updating the sidebar templates here: https://github.com/jupyter/jupyter-alabaster-theme/blob/master/jupyter_alabaster_theme/__init__.py#L35
• add requirement of using only one toctree in documentation
• installation/configuration instructions can change. It's easier to maintain if we put them in one spot