Add SVG style guide

Following up with https://github.com/swcarpentry/lesson-example/issues/171 and https://github.com/swcarpentry/styles/pull/203, we might need to clean up SVG files until Firefox or LibreOffice developers fix some issues, see links in the issue link before. I make [a suggestion](https://github.com/swcarpentry/styles/pull/203#issuecomment-365339435) and would be great to document it in http://swcarpentry.github.io/lesson-example/04-formatting/index.html or http://swcarpentry.github.io/lesson-example/06-style-guide/index.html. It might have a better place but I couldn't remember now.