Add yamllint to project

What does this MR do and why?

Mostly to test how possible it is to build up the project with tooling that:

• Is pinned in development and production versions.
• Is made up of lots of independent jobs, rather than requiring the team to maintain a monolithic Docker image.

Add yamllint to the project.

Also in this MR, make the pipeline shorter by not waiting for build stage to finish before starting tests (no tests need the build, yet).

How to set up and validate locally

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Check out this branch.
3. Run make yaml-tests. There should be no errors.
4. Remove the first line (---) from .gitlab-ci.yml and data/products.yaml.
5. Run make yaml-tests. Problems are detected in a file in the root of the project, and also in a subdirectory.

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.