feat: document two-step registration

[hperl]

Screenshot

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

[aeneasr]

Could you add screenshots please?

[hperl]

Could you add screenshots please?

I already added it to the docs, and now also to the issue description :)

[vinckr]

lgtm