feat: document FedCM

[hperl]

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 security@ory.sh) 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

[christiannwamba]

Looks good to me.

[piotrmsc]

few minors but looking good :)

1. Please add short description on why it might be useful for the users - provide example description like with content page.
2. Check typos
3. I think then CNAME should be some sort of callout as prerequisite