Document how to add new node to public federation

[surchs]

After neurobagel/planning#104 we may have more people try out our tools and maybe also try a local deployment. We should tell them how they can make the federation API aware of their new node in that case:

• add to the docs how to add my new node
• discuss / decide a process by which the federation API checks if nodes are still alive

[github-actions]

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[surchs]

We are not going to focus on this right now because we have yet to see a demand for adding public nodes from users. Once that's confirmed we will address the issue