Restructure "Federating Your Data" docs to discuss philosophy of namespaces

[jhiemstrawisc]

@aowen-uwmad has requested more structured docs pertaining to how to pick a federation namespace for new origins. I'd like to use this as an excuse to do some restructuring for our "Federating Your Data" documentation section.

Right now our ToC has something like:

Federating Your Data: --> page describing step by step how to set up an origin, assuming Posix.
    S3 Backend --> page describing S3 backend specific information
    Generating Tokens --> How to generate various tokens

I'd like this to instead be something like:

Federating Your Data: --> page describing philosophy (origins, their roles in a federation, how to choose a namespace, etc)
    Setting Up An Origin --> Page with step-by-step instructions for generic origin configuration
    Posix-Specific Backend Considerations --> Talk about user mapping, etc. Justin needs help with this.
    S3-Specific Backend Considerations --> page describing S3 backend specific information
    HTTPs-Specific  Backend Considerations --> First pass at documenting this. Maybe even some docs about globus?
    Origins and Token Management