Markdown Style Guide for about.GitLab.com

Merged username-removed-236961 requested to merge handbook-markdown into master

Closes https://gitlab.com/gitlab-com/www-gitlab-com/issues/680 and #666 (closed) and #679 (closed)

Adding Kramdown (magic) guide to our Handbook/Marketing/Dev-Rel/Tech-Writing/ to facilitate writing content for GitLab's website and blog.

cc/ @axil @AshleyS @amara