docs: added metadata and auth flow

[himazawa]

No description provided.

[BenitoVisone]

I suppose that in the auth_flow diagram we're missing two interactions between steps 6 and 7:

1. update SAMLSession record with the SAMLResponse parameter after the redirect POST on the saml/acs endpoint
2. init OIDCSession record after successful issuing of the authorization_code

chore: maybe it could be useful to add the /token POST request after the step 7. to make init of AccessTokenSession Record more explicit

[BenitoVisone]

1. In the auth_flow I would connect the /token to the API Gateway instead of directly to oneid-ecs-core
2. In the metadata_flow I would add the Parameter Store dependency because the lambda will need it to sign and public the metadata with the certificate

[sebbalex]

@himazawa @BenitoVisone I think we're all set here, can we merge it?