Docs review and guidelines for the gitlab-ui project

For #598 (closed) and technical-writing#108.

@marcia will help to draft outlines and a doc template so that docs changes to this project can be reviewed by maintainers and the tech writers don't become a bottleneck. She is the go-to person for questions and advice in regards to documentation for development guidelines and will keep helping to document things in this project upon request.

• Maintainers of this project: provide an example of what a good doc here looks like (higher-level outlines and the minimum info every doc must contain);
• Marcia: create minimum guidelines for their docs;
• Marcia: create a documentation template.