Reorganize user guidelines into a dedicated section

[choldgraf]

Context

Right now we have a single About mybinder.org section that is broken down into "service information" and "project / org" information. In addition, we have a long-ish and growing "user guidelines" section that has a lot of useful in formation in it (like security! ref #263 ). That page is getting a bit long, and it can be hard to find information buried under many headers.

https://mybinder.readthedocs.io/en/latest/about/user-guidelines.html

Proposal

I propose that we try re-organizing the top-level About section into two sections: a Service guidelines section about the service itself, and an About section about the project / meta stuff / etc. This might simplify the top-level sections a bit so it's clearer what information is where, and also make it easier to find things