Document the meanings of columns in the exported CSV files

[capnrefsmmat]

It would be great if somewhere in the Export Data interface, perhaps when you click the CSV button, it would provide a summary of the column names and what they mean.

User story

A non-technical user decides to download our data to make a spreadsheet or graph. They press the CSV button and get a file with columns like "geo_value", "time_value", "value", "lag", etc., and don't know what these mean.

Additional context

We have documentation in several places that could be used here:

• API docs
• R package return value docs
• Python package return value docs

[capnrefsmmat]

This may also require a way of pointing to the API docs defining what a particular signal means.

[sgratzl]

the current version has links to the aforementioned api docs and signal descriptions, e.g. for python: