add sequence diagram to OID4VP

[Kristina Yasuda]

for clarity

[Paul Bastian]

This might be too much, but maybe it is a starter?

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[Paul Bastian]

please check it and let me know if that helps in any way

[Kristina Yasuda]

PR #464 was merged - it introduced two diagrams for same-device and cross-device flow based on this input. They are kept high-level because they are in the overview section. there is currently no diagram based for does not talk about authorization code flow.