Document how to update or grant more permissions to a created GraphDB user

[alyssadai]

As we discovered in neurobagel/planning#44, it appears that if you grant read/write permissions for a database to a new user using a GraphDB API call (i.e., see https://neurobagel.org/infrastructure/#create-new-database), there is no easy way to then update or add access permissions for additional databases to that same user without rewriting the entire list of {"grantedAuthorities"}. Basically, this means that every time you update a user, you are rewriting its permissions.

This is pretty annoying and unintuitive, so we should

• document the behaviour / how to grant access permissions for >1 db to a single user
• look into whether there is a reasonable workaround for additive permission granting (as well as removing permissions)
• include the curl command to check the settings for a GraphDB user, similar to the one we have for Stardog

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