It's very hard for end user to know what version of Gitaly is running,
and they should not have to care about this detail either. Gitaly 1.21.0
was released before 11.9 was tagged, so it's an unsupported version too.

This change removes the reference.

Add trouble shooting sections

This commit adds a documentation for parallel pipelines on merge
trains.

Fix typo

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

- Move images to each corresponding img/ folder