docs: add section for user's renku-env

[emmjab]

Is your feature request related to a problem? Please describe.
Functionality was added to allow users to create private renku-env repos that specify user-specific (as opposed to project-specific) configurations (i.e. .bashrc). This feature will eventually have a UI, but it needs documentation in its current form.
(SwissDataScienceCenter/renkulab-docker#65 & SwissDataScienceCenter/renku-ui#639)

Describe the solution you'd like
Clear and concise documentation for how users can add user level configs that apply to all projects.