Some admin_area docs were missing the Core Only badge

Some markdown headers needed tweaking to adhere
to standards, including blank lines above and below,
only one space after hash, first header should be
h1, and only one h1 per doc

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 makes other minor Markdown fixes that were near the main fixes.

This reverts merge request !17257

Setting `usage_ping_enabled` to false in `gitlab.yml`:

1. Disables the usage ping, regardless of the value stored in the database.
2. Prevents the usage ping from being enabled through the admin panel. It can
   only be enabled by either removing the line from `gitlab.yml` and configuring
   through the admin panel, or setting it to true in `gitlab.yml`.