docs: Improve preview deployments documentation with reference to variables

[Bredo]

Added information on referencing generated domain in environment variables for preview deployments.

Greptile Summary

This PR adds a small informational callout to the preview deployments documentation, pointing users to the ${{DOKPLOY_DEPLOY_URL}} service-level variable when they need to reference the generated domain in their environment variables.

The change is accurate — the variable name and syntax (${{DOKPLOY_DEPLOY_URL}}) match what is documented in variables.mdx, and the anchor link (/docs/core/variables#service-level-variables) correctly points to the relevant section.

Key observations:

• The variable syntax and name are consistent with the existing variables documentation.
• The anchor link target #service-level-variables maps to the ## Service-Level Variables heading in variables.mdx.
• The callout is placed after the wildcard DNS paragraph in the "Custom Domains" subsection, but DOKPLOY_DEPLOY_URL is relevant for all preview deployments (including the default traefik.me domains), not just custom-domain setups. Consider moving it to the top of the ## Configuration section for better discoverability.

Confidence Score: 5/5

• This PR is safe to merge — it is a small, accurate documentation addition with no code changes.
• The only change is adding a documentation callout. The variable name, syntax, and link target have all been verified against the existing codebase and are correct. No functional code is modified.
• No files require special attention.

_{Last reviewed commit: c0733d6}

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