Port external link checks from 'gitlab-docs' project

What does this MR do and why?

Port existing external link checks from gitlab-docs project so we can start to check external links again with docs-gitlab-com:

Test jobs:

• charts: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/9355782082
• gitlab: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/9355782079
• runner: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/9355782081
• omnibus_gitlab: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/9355782080
• operator: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/jobs/9355782083

Related to #270 (closed)

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