Markdown items: mermaid diagrams, kbd styling

I know @jeldergl has been building a library of visual components we use, and I've identified two that (mostly) appear in the documentation but generally NOT the GitLab application:

• Mermaid diagrams.
  • See https://docs.gitlab.com/ee/topics/gitlab_flow.html#github-flow-as-a-simpler-alternative for an example
• <kbd> tag styling of text
  • See https://docs.gitlab.com/ee/user/shortcuts.html in the documentation
  • Press ? on this page to see it in the application.
  • @mle created gitlab-com/www-gitlab-com!100806 (merged) last week to add styling for release post items, so there's precedent for this styling being missed

I'm not sure how to categorize this issue, but I trust that either Jeremy or Michael will.