Implement and document server-side configuration

[krassowski]

The configuration file with links to repositories, descriptions, etc can be either a custom toml/json/yaml file or a traitlet (py/json file); the question here boils down to preferences on file format, and:

• if we use a traitlet we can re-use the jupyter-server config files (py/json); the python files could read in the PAT token from environment variables and embed it in the repository URL used for cloning
• if we use a custom file (toml/yaml/json), we will need to implement variable substitution

Originally posted in nebari-dev/nebari#2456 (comment)

Note: It cannot be placed in a JupyterLab JSON overrides as these are client-side and would expose PATs

[krassowski]

There needs to be a way to setup it via nebari config; if it is a traitlet, then maybe nebari should have a sibling entry to jupyterlab.default_settings but for the server-side settings.