enable Mintlify preview environments

[dangrondahl]

Mintlify Pro supports per-PR preview deployments, giving reviewers a live
preview of documentation changes before merging.

Tasks

• Confirm Mintlify Pro is active for the `kosli-dev` org
• Enable preview environments in the Mintlify dashboard
• Verify that opening a PR generates a preview URL
• Add the preview URL to the PR description template if supported

References

• https://mintlify.com/docs/deploy/previews