docs: developer guide to integrate a new provider

[slint]

• Debugging (breakpoints, logging)
• What kind of settings to expect from OAuth providers (e.g. client key + secret)
  • What to expect from OIDC providers
• Explain the entire function call flow in terms of invenio_oautclient.handlers
• What happens when not all required attributes are returned
  • How to make sure the user never goes through a registration form
• Picking a unique external identifier from the provider to link to the User
• What is the expected dictionary structure of the account info handler?
• Template (cookiecutter?) for new contrib providers