Bring the example config and the main doc together to simplify
documentation for code quality

Image converted to reduce file size.

Let's use disqus_identifier since this is a more
descriptive name. We will need redirect_from for
another purpose anyway (Nanoc redirects).

Previously, we used brackets to denote the tier badges,
but this made Kramdown, the docs site Markdown renderer,
show many warnings when building the site. This is now
fixed by using parentheses instead of square brackets.

This was caused by [PREMIUM] looking like a link to
Kramdown, which couldn't find a URL there.

See:
- https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/484
- https://gitlab.com/gitlab-org/gitlab-ce/issues/63800

- Edit pages under user/project/merge_requests and
  add the relevant types in the frontmatter.
- Clean up descriptions.

Based on https://gitlab.com/groups/gitlab-org/-/epics/1280