Move details of markdownlint mostly to the documentation
index for a SSoT, and link apropriately

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

Many code blocks are 4spaced, and they render in GitLab
without coloring as a result, even though they are
fenced with a language label. If in a list, other items
will render as being in a code block too, even if not
meant to. This fixes all these issues for most docs in
/development, and cleans up other whitespace issues too

Search and replace gitlab-com/gitlab-docs with gitlab-org/gitlab-docs.
For https://gitlab.com/gitlab-com/gitlab-docs/issues/310

Evan Read authored 5 years ago and Achilleas Pipinellis committed 5 years ago

Also adds and one linting rule
and makes project conform to it.

Previously it was unclear if the docs changes should be included in
the MR as well, and adding a changelog entry in the CE MR also triggers
the ee-specific-lines-check job in CI.

This is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/61945