Add a description of the config file and using the attrs library to the docs page

[genevievestarke]

Add a description of the config file and using the attrs library to the docs page

The documentation current talks about some aspects of adding a new model, but it does not describe how to use the base config or mention the attrs functionality that we use in H2I. We should add a description of how the configs are used in H2I, and an example of how to use attrs to manipulate the inputs (e.g. attrs post init), plus link to some attrs docs.

Proposed solution

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