Add "data flow in Site Kit" documentation.

[tofumatt]

Summary

Addresses issue:

• #3454.

Note: this should probably be merge reviewed by @aaemnnosttv, @eugene-manuilov, @felixarntz, and/or @techanvil before merging, just to ensure accuracy and that we didn't miss anything 😄 .

Relevant technical choices

While this largely covers the sections in the IB, it's already a lot to review so I wasn't too detailed on the REST API side of things, mostly covered the lifecycle and reasoning why we do things this way (Dashboard Sharing, for instance).

I didn't really have great context for the setup_services method, as there's not a lot to really explain there right now. Maybe someone else wants to write a summary for it? But give it's mostly about listing relevant permissions/scopes I wasn't sure what to write. Maybe that's enough? 😅

PR Author Checklist

• [x] My code is tested and passes existing unit tests.
• [ ] My code has an appropriate set of unit tests which all pass.
• [ ] My code is backward-compatible with WordPress 4.7 and PHP 5.6.
• [ ] My code follows the WordPress coding standards.
• [x] My code has proper inline documentation.
• [x] I have added a QA Brief on the issue linked above.
• [x] I have signed the Contributor License Agreement (see https://cla.developers.google.com/).

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Code Reviewer Checklist

• [ ] Run the code.
• [ ] Ensure the acceptance criteria are satisfied.
• [ ] Reassess the implementation with the IB.
• [ ] Ensure no unrelated changes are included.
• [ ] Ensure CI checks pass.
• [ ] Check Storybook where applicable.
• [ ] Ensure there is a QA Brief.

Merge Reviewer Checklist

• [ ] Ensure the PR has the correct target branch.
• [ ] Double-check that the PR is okay to be merged.
• [ ] Ensure the corresponding issue has a ZenHub release assigned.
• [ ] Add a changelog message to the issue.