docs: Add to the documentation how to get the REGISTRATION_KEY to be registered as a user on sign-up page

[DeaMariaLeon]

I spent a considerable amount of time to sign-up to a local Conbench's server.
This issue didn't help, as the REGISTRATION_KEY turned out to be "innocent-registration-key"

I suggest adding or updating the instructions to the docs for this as the key here: https://github.com/conbench/conbench/blob/main/conbench/config.py doesn't match.

[austin3dickey]

Thanks for the issue!

As you have found, the "new Conbench deployment" administration experience is in a horrible state right now. Thanks for being a trailblazer and reporting the issues you find. I'm excited to make it better.