Removes duplicated information, and combines sections and files
together when they are duplicated or highly related. Also adds
intros to all pages and sections missing intros, and troubleshooting
sections to pages that may need them in the future. Links more to
other docs, and clarifies language when abosolutely necessary, and
adds angle brackets to sample parameters.

Correct the spacing of unordered markdown lists
in docs, to maintain standards of documentation.

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