Add arc diagrams to docs

[pingbird]

Adds diagrams that were merged in flutter/assets-for-api-docs#197

[flutter-dashboard]

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If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[chinmaygarde]

cc @gspencergoog

[jonahwilliams]

Just dropping by to say that these diagrams are awesome, thank you for adding them @PixelToast