Fix duplicate titles and descriptions

Follows !256 (merged) !257 (merged)

From the data available in Google Search Console, we need to eliminate two things:

• Duplicate titles (do it manually) => https://gitlab.com/gitlab-org/gitlab-ce/issues/46061
• Duplicate descriptions (do it dynamically): for this one, I suggest we pick up the content of the first paragraph as the fallback to the page's description:. Currently, the fallback is plain text repeated in multiple pages. This will be the second iteration of the meta description added a while ago. => Some added by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18789, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5603 till we resolve https://gitlab.com/gitlab-com/gitlab-docs/issues/200

cc/ @JobV @axil I suggest we add this to the Epic gitlab-com&4 (closed)