Complete Tech Writing Handbook
This MR completes the Technical Writing Handbook: https://about.gitlab.com/handbook/marketing/developer-relations/technical-writing/
Going along with https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/2373, closes the gaps related to instructions on how to write for GitLab (including about.GitLab.com and our Blog).
This MR includes an adaptation of https://gitlab.com/gitlab-com/blog-posts/blob/master/STYLEGUIDE.md and concepts from http://docs.gitlab.com/ce/development/doc_styleguide.html, also used together on https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/2373 and https://about.gitlab.com/handbook/marketing/product-marketing/content-marketing/#blog