doc: update authentication and grant consent diagrams

[kleyow]

:thinking: Added the invalid OTP flow to diagram.

Unsure if I need to add more edge case errors like

• dfsp backend returns invalid payload which doesn't conform to

  { isValid: true|false }

• No scopes being returned from dfsp backend.

Since we don't validate backend requests payloads atm.

[lewisdaly]

thanks Kevin. Can you please make this a separate puml file, and explicitly add a new section to the linking document (maybe section 4, Linking Error Scenarios)? That makes this documentation much more readable.

[kleyow]

@lewisdaly lmk if this is better.

[lewisdaly]

Yep, thanks @kleyow