Move shebang into its own subchapter

In rust-lang/reference#2192, we extended our documentation about
shebangs but kept it in Input Format under a section on shebang
removal.  But really, this section is mostly now about documenting
what a shebang is, for our purposes.

Let's separate this out into a new subchapter.

We'll keep the rule about shebang removal in Input Format; it can link
to Shebang for a description of what a shebang is, its grammar, etc.

This follows the pattern that we'll similarly be using for
frontmatter.