doc: Add "A 30-minute Introduction to Rust"

By Steve Klabnik.
doc: Edit intro
Only check --crate-type flags if present.

Before, normal compilation and the --crate-file-name flag would
generate output based on both #![crate_type] attributes and
--crate-type flags. Now, if one or more flag is specified by command
line, only those will be used.

Closes #11573.
doc: Address feedback about intro
auto merge of #13416 : brson/rust/30min, r=alexcrichton

This is intended to be the first thing somebody new to the language reads about Rust. It is supposed to be simple and intriguing, to give the user an idea of whether Rust is appropriate for them, and to hint that there's a lot of cool stuff to learn if they just keep diving deeper.

I'm particularly happy with the sequence of concurrency examples.
auto merge of #13489 : JustAPerson/rust/crate-file-name, r=alexcrichton

Before, the `--crate-file-name` flag only checked crate attributes for
possible crate types. Now, if any type is specified by one or more
`--crate-type` flags, only the filenames for those types will be
emitted, and any types specified by crate attributes will be ignored.