Rollup merge of rust-lang#29538 - steveklabnik:ch1_edits, r=brson

I did some preliminary editing work with No Starch on the first chapter of the book, and here's some of the results. We're going to want to return to this later when @brson etc's new rustup work is done, so this is mostly just a first pass.

But, we agreed that having separate chapters for each of this bit of intro is a bit excessive. So let's move all of this intro stuff into one chapter.

I'd appreciate a careful review of this, as there was also some confusion about some things, which resulted in me taking one huge markdown file apart and splitting it back up, as well as some editor issues, so I _think_ this looks good, but double checking things matters!

/cc @aturon

Author: steveklabnik

src/doc/trpl/SUMMARY.md

@@ -1,9 +1,6 @@
 # Summary
 
 * [Getting Started](getting-started.md)
-    * [Installing Rust](installing-rust.md)
-    * [Hello, world!](hello-world.md)
-    * [Hello, Cargo!](hello-cargo.md)
 * [Learn Rust](learn-rust.md)
     * [Guessing Game](guessing-game.md)
     * [Dining Philosophers](dining-philosophers.md)