Rewrite ancillary uses to focus on 2 kinds of ancillary APIs rather than ancillary data.

[jyasskin]

This is a replacement for #359 after the discussion in https://github.com/w3ctag/privacy-principles/blob/main/meetings/2023-10-04-minutes.md. It will close #220.

@pes10k, I think I fixed your comments:

• "identifiable person" was an echo of a phrase in the definition of "personal data", but I found a way to avoid it.
• I replaced the CSP report example with event timings. There's also a nit there that the duration field is a novel piece of data, so I could narrow this to just the processingEnd field.
• I reworded the "it complies with this principle" bit and also moved the second principle in the section down to just before the paragraph that elaborates on it. This violates the guideline we've been following to put principles at the tops of their sections, so maybe folks will want me to move it back.

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[pes10k]

From slack

But if @torgo (or others) supports that direction again, as an alternative to the “yes/no exists and not planned for removal in existing APIs” way of splitting things up, then I’d like to revist```