Fix outdated security documentation

[colinpalmer]

The security documentation at https://jupyter-server.readthedocs.io/en/latest/operators/security.html is out of date.

It has a section that describes an additional one-time token used for browser launch, but this behaviour was replaced by a redirect file a few years ago in https://github.com/jupyter/notebook/pull/4260.

Please could the docs be updated to describe the current implementation? Thanks!