Update documentation to clarify the Death table standard

[AndyRae]

Is this the right issue type?

• Yes, I'm planning work for this project team.

Summary

Our approach is that the user needs to shape their data to be in the Death Table standard.

We document that this requires death_date as a column name

Add a new section to make it clear to users that the data standard for Death is specific.

https://carrot.ac.uk/standards

Acceptance Criteria

• Documentation added.

Tasks

• Documentation.

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