Document the integration between Rafiki, OpenAPI Spec & openpayments.guide

[AlexLakatos]

[sabineschaller]

When a PR to change the spec is merged into the open-payments repo, the changes show up on openpayments.guide automatically (done via github actions). To update the OpenAPI spec validator within Rafiki, update the link to the spec file in packages/backend/src/config/app.ts. Make sure to use the url with the commit hash and not main. You will then need to go and change the code in packages/backend/src/open_payments to reflect the spec changes.

[sabineschaller]

Same goes for the auth package.