chore: remove documentation about old (internal) destination API. Add remark about destination / keys duplication (planning#1788)

Endpoints:
  - GET /v1/destinations/{destinationId}
  - GET /v1/destinations/{destinationId}/keys/{keyId}
  - GET /v1/info
  - GET /.well-known/jwks.json

See https://swagger.io/specification/#media-type-object

...which must have 'in' and 'name' keys

This allows backwards compatible changes in the future.

The `X-` prefix is deprecated as of https://datatracker.ietf.org/doc/html/rfc6648