Add a link to Jupyter Server config documentation to Scheduler’s "Server configuration" docs section

[andrii-i]

Problem

Issue #498 mentions not being able to find guidance on server configuration via config file in Jupyter Scheduler docs.

Proposed Solution

Add a link to Jupyter Server config documentation (https://jupyter-server.readthedocs.io/en/latest/other/full-config.html#config-file-and-command-line-options) to Scheduler’s Server configuration docs section (https://jupyter-scheduler.readthedocs.io/en/latest/operators/index.html#server-configuration) to have an explicit guidance.