message: docs: Document data migration actions going from v2 to v3
diff: This is important for anyone who starts the server from code.
  - When logging in, a consent screen will now provide information about the client.
  +### Data migration
  +The following actions are required if you are upgrading from a v2 server and want to retain your data.
  +
  +Due to changes in the keys used by the IDP, you will need to delete the stored keys and sessions.
  +If you are using a file backend, delete the `.internal/idp/` folder in your data folder and restart the server.
  +This will not delete the user accounts, but users will have to log in again.
  +
  ### Configuration changes
  You might need to make changes to your v2 configuration if you use a custom config.
  