Google Console errors

Follows https://gitlab.com/gitlab-com/gitlab-docs/issues/167

Site errors:

Besides duplicate titles and descriptions (will be solved by #194 (closed)), Google Console points to these errors:

• 404 and 205: check page by page presenting errors and fix them
• Structured metadata: restructure to have 1 org (home), 1 website (home), multiple webpages (all of them) => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/261
• Mobile usability errors: fix them individually
• Blocked Resources: not sure how to read this data 🤔
• Check the sitemap again

cc/ @axil @JobV