Remove Disqus from docs.gitlab.com

Related to: https://gitlab.com/gitlab-com/marketing/inbound-marketing/growth/-/issues/1135#note_1264438714

Steps to retire Disqus for docs.gitlab.com:

• 1. Remove the embed code for Disqus (!3682 (merged)).
• 2. Remove Disqus documentation (gitlab!115213 (merged) and technical-writing#779).
• 3. Remove comments and disqus_identifier front matter lines from documentation markdown pages in each repo, as these will no longer have any affect.
  • gitlab: gitlab!115477 (merged)
  • gitlab-runner: gitlab-runner!4008 (merged)
  • omnibus-gitlab: omnibus-gitlab!6770 (merged)
  • charts: none
  • operator: none
• 4. Remove the Zapier integration from the #docs-comments Slack channel (technical-writing#779 (comment 1331653244))