Add "Getting started" docs route for users who already have JSONLD data

[alyssadai]

This entails:

• Adding/updating a section on the "Maintaining a node" page to tell users how to change the following, in addition to LOCAL_GRAPH_DATA (as these changes are intuitively linked when updating or configuring a first toy deployment to use real data)
  • NB_GRAPH_USERNAME
  • NB_GRAPH_PASSWORD
  • NB_GRAPH_DB
• On the "Getting started" page, add/update an info box with something like "If you already have generated Neurobagel graph-ready data (JSONLD files), follow the steps to configure your deployment for your data before launching the stack" that links to the section in the first bullet point
• Similarly, at the end of the "Getting started" page, re-link to that section again

[github-actions]

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[surchs]

Making this a someday issue as we have much improved the getting started docs