docs: add KRATOS_ADMIN_URL env variable and fix HYDRA_ADMIN_URL mistake

[emmanuelgautier]

Checklist

• I have read the contributing guidelines and signed the CLA.
• 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 the necessary documentation within the code base (if appropriate).

Further comments

Add more documentation for Hydra consent flow integration.