Update PROTOCOL to include change data spec

[nkarpov]

Description

Update PROTOCOL.md to include change data file spec.

I think it's possible to consider these new change files as "data files", but I've documented them as their own file type to start because they do not represent the actual table data the same way add and remove files do.

How was this patch tested?

N/A

Does this PR introduce any user-facing changes?

Yes. This PR introduces changes to the documentation of the Delta Lake protocol

[scottsand-db]

@tdas any last comments?

[zsxwing]

@nkarpov left a few minor comments. We are still missing a section to explain how change data readers generate results. We can do that in a followup PR.