Document dev-only configuration options for Neurobagel tools

[alyssadai]

We have removed some variables from being configurable in the default deployment recipe in neurobagel/recipes#80; however, these variables are still used internally by individual Neurobagel services and can be set locally when running the tools in a dev environment.

We should document these variables somewhere outside of the https://neurobagel.org/config/ page.

[github-actions]

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