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

All code blocks should be surrounded by blank lines

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

After review, heading was changed and links to
it needed to be updated, and minor tweaks to
the issues docs such as note formatting

Ensures we give instructions for how to create ssh keys
in a single place only.