docs: passwordless sms login

[aeneasr]

Related Issue or Design Document

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

[piotrmsc]

nit: below there is no registration view, only direct new configuration section you have added and above there is only a sign-in screen example so the old text does not match the current layout.

[piotrmsc]

nit: do w have any (our) documentation/blogpost that we can link as warning explaining why one should not use OTP via text message?

[aeneasr]

Thanks @piotrmsc - addressed your feedback

[vinckr]

lgtm