Split README into multiple docs

[ml8]

The README is huge and has multiple audiences. Split the doc up to make it less intimidating and tailor it for each category of users. Suggestion:

• Overall README describing repo and pointer to docs.
• Description of Jupyter/JupyterHub model (hub + servers).
• HOWTO for faculty who don't want to know how this deployment works.
• Description of kubernetes model for Jupyter/JupyterHub (i.e., what we have now/focused version of ztjh docs).
• HOWTO set up environment for development.
• Description of config files and how to update configs and deploy new versions.
• Maintenance + troubleshooting guide

[ml8]

@anesepark and @walidabualafia can you do this in the spring please?