Document how to group things for GCP and GitLab product factories and GKE runners

Following on conversations with @rk725, @ad2139, @rjg21 and @ek599, add some guidance on the whole "gcp-product-factory", "gitlab-product-factory" and GKE runners config.

At a high level:

• HOWTO for "I want to start a new product and there should be one of those for everything" (!279 (merged))
• HOWTO/checklist for "I want to migrate an existing product" (!279 (merged))
• Explanations for:
  • How do I group things/what are "products" in each of those cases. (!279 (merged))
  • How should groups/etc be created in GitLab (!279 (merged))
  • Where things should live (!279 (merged))
• Reference on what each sets up and how they depend on each other. (!279 (merged))

As a summary from discussions:

• Group Google "products" as makes sense for sharing GCP resources. (E.g. if one has multiple GitLab projects which all define a set of inter-connected components within a single GCP deployment, they live in the same Google product.)
• Group GitLab "products" as makes sense for development. (E.g. if one wants to see all issues from a set of GitLab projects as one, or manage IAM permissions as one, put them in the same group and, hence, same GitLab product.)
• Group GitLab GKE runner "products" as makes sense from a CI IAM perspective. (E.g. CI jobs in a set of projects all need to be able to access the same Google resource/secret, group them into the same runner "product".)