Add Markdown linting and one rule

Runs Markdown linting on project, for a single rule to start with.

https://github.com/markdownlint/markdownlint/blob/master/docs/RULES.md#md032---lists-should-be-surrounded-by-blank-lines

In some cases, breaking this rule breaks the HTML renderer (which is the case this project in many places).

Relates to: https://gitlab.com/gitlab-com/gitlab-docs/issues/287.

Related to https://gitlab.com/charts/gitlab/issues/1529

@WarheadsSE @twk3 @axil Could one of you review and merge?