Add rustpkg tutorial to the official tutorials.

Three things in this commit:

1. Actually build the rustpkg tutorial. I didn't know I needed this when
   I first wrote it.
2. Link to it rather than the manual from the
   tutorial.
3. Update the headers: most of them were one level too deeply
   nested.
auto merge of #9439 : steveklabnik/rust/build_rustpkg_tutorial, r=brson

Three things in this commit:

1. Actually build the rustpkg tutorial. I didn't know I needed this when
   I first wrote it.
2. Link to it rather than the manual from the
   tutorial.
3. Update the headers: most of them were one level too deeply
   nested.
Merge branch 'master' of https://github.com/mozilla/rust