Correct the spacing of unordered markdown lists
in docs, to maintain standards of documentation,
in /user and others.

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

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

Remove redundancies:
- Domains
- User/group/project sites

GitLab Pages supports projects hosted under a subgroup, but not subgroup
websites. That means that only the highest-level group supports

i.e.:
You created a group for your engineering department called
`engineering`, a subgroup for all your documentation websites called
`docs`,and a project within this subgroup is called `workflows`.
Your project URL is `https://gitlab.com/engineering/docs/workflows/`.
Once you enable GitLab Pages for this project, the site will live under
`https://engineering.gitlab.io/docs/workflows`.

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Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/1333

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