Align images and lists in assorted markdown docs

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

Change assorted absolute links to ce and ee docs
to relative links

Fix more links after review

Update redirect_from link

Apply suggestion to doc/ci/introduction/index.md

Apply suggestion to doc/ci/README.md

Apply suggestion to doc/ci/README.md

Make front matter redirect_from links absolute

Change front matter to fix discus comments

Implements part of the single source of truth audit
fixes for the CI section.