Add docs for how to contribute to the mkdocs

Copy https://github.com/openboxes/openboxes/tree/develop/docs#contributing into a new section here: https://openboxes.gitbook.io/contributor-guide/writing-documentation

Then add some additional info:
- how to edit existing pages
- how to add new pages (remember to add them to the nav/sitemap
- how to suggest/request new things to be documented