Add lint checker for blog posts categories
Based on the recently updated https://about.gitlab.com/handbook/marketing/blog/#categories, we have fixed categories that we need to use.
This MR introduces a simple lint checker for the said categories.
Proof that it works https://gitlab.com/gitlab-com/www-gitlab-com/-/jobs/68057877. We need to fix all those posts before merging.
Edited by Achilleas Pipinellis