Enforce consistent prefix for bullet lists
What does this MR do?
Enforced another Markdown rule: https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md004---unordered-list-style, which aligns with our Styleguide for bullet lists: https://docs.gitlab.com/ee/development/documentation/styleguide.html#list-items.
Bullet lists should always be prefixed with -
.
Relates to: https://gitlab.com/gitlab-com/gitlab-docs/issues/287.
CC @gl-docsteam