Add tutorials pages

This adds a tutorials menu page, a seven-page getting started series, a formatting tutorial, tips and tricks page and an 'other documents' page with links, along with associated images for the 'getting started' series.

There is also a new link in application.html.erb to the tutorials menu page. I have also commented out the 'about' link in that file, because I don't feel it's necessary (it's a page only linked to from the "Find out more" buttons on the index page, and that's the way people will be finding that page - it's not a stand-alone page. Also, once other items such as the blog and planet are added to that menu, it will start to get a bit full.

I've added two new classes to main.css.scss, which are used in the tutorials.

I've also added links to these pages in routes.rb - hopefully I've done this correctly - it's something I'm not familiar with.

Things which need to happen before this is merged:

1. Update some of the images. @DeadSuperHero offered to help with this - Sean could you contact me to discuss?
2. Anyone who wants to, please check the 'other_docs' file and update/add any links you think should be there.
3. The tips file is not currently linked from any page. It needs work from someone else to add a few more tips before it is worth adding to the site, and then a link to it added to the 'Other tutorials' menu of the tutorials page.

I've done as much as I can on these. The getting started series is, I'm pretty sure, complete, apart from the few images which need updating. However, feel free to make any changes to these pages you feel are necessary.

It would be great if this could be made ready to be merged in time for the anniversary on 27 Aug.