feat: docs for b2b sso

[hperl]

Related Issue or Design Document

Cloud PR: https://github.com/ory-corp/cloud/pull/5301

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

[vinckr]

Looks pretty good,
a few stylistic improvement suggestions

all addressed :)