Rework 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.
Start a 'general' tutorial section, along with some initial setup instructions.
Merge pull request #133 from creature/gh-pages

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