docs: add KRATOS_ADMIN_URL env variable and fix HYDRA_ADMIN_URL mistake

[emmanuelgautier]

Checklist

• [x] I have read the contributing guidelines and signed the CLA.
• [x] I have read the security policy.
• [x] 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.
• [x] I have added the necessary documentation within the code base (if appropriate).

Further comments

Add more documentation for Hydra consent flow integration.