Doc: Explain how to use 2FA with API

[bkmgit]

This is described at https://talk.desec.io/t/does-api-access-circumvent-2fa/759 but should also be in the documentation. The documentation can be updated if the api changes and a warning can be included that the api is not yet stable.

Can create a pull request.

[peterthomassen]

We do not document API endpoints when they are not stable.

Nevertheless, I've added some of the information you linked (d79abf5).

[bkmgit]

Thanks.