Check if the product is present and hide the topics if not

We had switched to using relative URLs for the topics of the landing page in the past in !734 (merged), but we then used full URLs again in !742 (merged).

The reason is that we use this project as a Docker image (registry.gitlab.com/gitlab-org/gitlab-docs:docs-lint) to build and test other projects like Omnibus, Runner, etc. When doing so, the GitLab project is not present, so the ee/ relative paths (and omnibus/ respectively) will fail the internal_links test we have in place.

With this change, we first check if the content is present and show the topics if so. We hide them otherwise, so that the internal_links test won't fail.

Replaces !1089 (closed)

All content	Omnibus missing	GitLab missing	Omnibus/GitLab missing