[ENH] Add example config for deploying a node behind NGINX and clarify data uploading

Changes proposed in this pull request:

Add new section "Behind a proxy" with example docker-compose.yml including NGINX+ACME
Restructure "Updating data in the graph" section slightly, and link to it more explicitly in the Getting Started steps, to further centralize and make intuitive the key steps for setting up a database. This:
- avoids the complexity of having to link to both https://neurobagel.org/getting_started/#the-neurobagel-node-deployment-recipe and https://neurobagel.org/maintaining/#updating-the-data-in-your-graph whenever we want to refer to a complete node setup (e.g., in this PR, following the old structure, it was difficult to tie in the customized NGINX docker-compose.yml back to the rest of the node setup steps without having to reference multiple different sections)
- provides a more straightforward workflow for users who completed the annotation process first (related: https://github.com/neurobagel/documentation/issues/202)
Differentiate names of sections relating to updating graph data to avoid confusion, and fix outdated links pointing to manual setup instructions
Fix some indenting in admonitions

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