Create parallel jobs for internal links testing

Why

If we bring the Docker image that is used by all projects to check for dead links to gitlab-docs, we need to have a way to test that if we change something in the Dockerfile or update Nanoc, that nothing will break in the downstream projects.

How

What needs to be implemented:

• Create 5 different jobs that will run in parallel

The workflow should be:

1. Internal links check starts in 5 parallel jobs, build cache is downloaded in all (can add as a dependency)
   1. The first job will test as before (tests site with all products present)
   2. The rest of the 4 jobs will test each product separately by setting CI_PROJECT_NAME to each project name.