Import changes to fix code blocks, etc.

[crandmck]

Import the changes made in https://github.com/expressjs/express-api-docs/pull/3. This allows us to use backticks for code blocks, and some other improvements.

[crandmck]

This includes the changes above and modiying all the code blocks to use backticks. I also added the Gemfile, updated config.yml as in https://github.com/expressjs/express-api-docs/pull/3, as well as adding to it the line

hard_wrap: false

which seems to be required to make text wrap properly.

However, in this branch there is an issue with the 4.x API doc page (http://localhost:4000/en/4x/api.html when running locally). It seems that prism.js is throwing an error, which really screws up the layout for that page (only).... but I haven't been able to get to the bottom of it.

@hacksparrow Can you please have a look and see if you can sort out what's going on?

Also, I would appreciate any other eyes. Merging this branch is going to be a huge PR because it touches almost every file.

[crandmck]

I figured out the problem with the 4.x API page and committed fix to the branch.

[crandmck]

Fixed with #628 for English content.
I did not do the same for translated content, so the PR wasn't as large as I originally thought.