metrics/data page needs to link to format docs

Periodically I tell people about datasets on the metrics page. But the text on the data page just says things are "sanitized", not how.

For example, in https://metrics.torproject.org/data.html#bridgeassignments what is the hash in the file?

We should document the formats of each of our data files, and especially point out where to learn about privacy-preserving changes we make.

We probably have much of this documentation already written, and scattered in various places. But it's not clear from the webpage where to look.