Update authentication documentation

The [authentication documentation](https://github.com/UUDigitalHumanitieslab/I-analyzer/blob/develop/documentation/Authentication-and-authorization.md) is out of date.

Start by reading documentation/Authentication-and-authorization.md and compare its statements with the repository's current authentication behavior. Update the document wherever it is outdated, then verify that it accurately describes the current authentication and authorization process.

Written by the indexing model from the issue text.