Rework HTML tutorial 6 and introduce a "general" section for tutorials.

This pull request contains two separate pieces of work, which is unfortunate. I couldn't figure out how to split them out into two pull requests.

- **Update the HTML tutorial 6**. This started as a simple grammar fix, but has ended up as a major rewrite. As well as fixing up the grammar and typos, I've tried to make this clearer based on the areas that I've seen students struggle with, as well as simplifying the language.
- **Create a 'general' section, with some initial setup instructions**. This is a new section, currently with one tutorial/howto with instructions on setting up a laptop for Codebar. In the future it could be used for other general topics that don't fit neatly into the existing structure. 
I would also mention rvm.