Add detailed (visual) explanation for member sign up.

[jbusecke]

Member Sign up still causes friction and relies too much on DCT members, which is not necessary. I will write some docs on the different steps and how to trigger/troubleshoot each one. This should serve as a base to instruct more folks at LEAP to feel confident to handle these tasks, and thus increase redundancy.

[RobertPincus]

Should include a link to the [LEAP Pangeo membership request form](LEAP Pangeo membership application).