Re-add conditional rendering for the homepage

What does this MR do and why?

This was mistakenly removed in !4461 (merged).

This conditional allows link checking to not fail on pipelines that do not include a full site build.

See also: gitlab-org/cloud-native/gitlab-operator!745 (comment 1761562359)

Screenshots, screen recordings, or links to review app

No visual changes.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Run make view
3. Verify that you can see the homepage
4. Recompile with an env var to simulate a non-docs pipeline: CI_PROJECT_NAME="other-thing" make view
5. Verify that the homepage is blank

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.