Reword tutorial

More of less the content, but better wording. Highlights:

* No initial header. Chapters don’t start with two headers.
* Recommend _Ruby in Twenty Minutes_ over _Programming Ruby_. The later is old and ugly.
* Add recommendation for a CLI crash course.
* Removed `Rakefile`.
* Removed use of `create-item`.
* Made section titles active (“Create a site” instead of “Creating a site”).
* Decrease “bear with us” attitude. The purpose of the files and directories after `create-site` are explaned right away, for instance. Also, the fact that nanoc uses absolute paths by default is now more explicitly stated.

To do:

* [x] Finish rewording the tutorial, obviously.
* [x] Ensure filenames and URIs are properly marked up (not only `code`).
* [x] Standardize on “forematter” instead of metadata section.
* [x] Remove `pre` titles. They’re not particularly useful (they are not shown and even if they were, they wouldn’t be selectable).
* [ ] ~~Use indented code blocks.~~ _Not possible everywhere_
* [x] Verify that what I’ve written so far is really correct.

Maybe:

* Play around with other markup languages. I am liking DocBook (but not the toolchain around it).
* Write a YAML tutorial. This one should cover only what is commonly used in the context of nanoc.
* Highlight identifiers more, because it is a common source of confusion.
* Use Bundler instead of manually installing gems.

Still a **work in progress** as I have only redone the first few sections.

CC @bobthecow @opensourcegrrrl