add redis cache document & fix a few maruku bugs

Redis documentations in the cache.md file. It's a simple configuration, so no need to make complex docs.

There was an issue with some <pre...> elements in the docs that maruku didn't like because they didn't have {% raw %} and {% endraw %} tags, I fixed those, so the CONTRIBUTING.md docs for contributing to the documentation works.

This can be merged at the same time as the redis cache PR I'm about to re-submit.