update to mkdocs.yml

Created by: joannechester

The syntax seems to have changed for loading some markdown extensions. Neither codehilite nor toc would load with the syntax - codehilite(guess_lang=false) and - toc(permalink=true) . Sytax that does work is

• codehilite: guess_lang: false
• toc: permalink: true We also need to change from using 'pages' to using 'nav' as pages has been deprecated. https://github.com/mkdocs/mkdocs/blob/master/docs/about/release-notes.md#the-pages-configuration-setting-has-been-renamed-to-nav