Rename files to better match mdbook generated sidebar numbering

I have had off-by-one errors when I'm talking to people and looking at
the file names, but they're looking at the sidebar in the generated
output, since the first section in a chapter doesn't get a number in the
sidebar, and the second section in a chapter gets named x.1, etc.

Just wait til I try to name cache invalidation.

src/SUMMARY.md

@@ -1,30 +1,30 @@
 # The Rust Programming Language
 
-- [Introduction](ch01-01-introduction.md)
+- [Introduction](ch01-00-introduction.md)
-    - [Installation](ch01-02-installation.md)
+    - [Installation](ch01-01-installation.md)
-    - [Hello, World!](ch01-03-hello-world.md)
+    - [Hello, World!](ch01-02-hello-world.md)
 
-- [Guessing Game Tutorial](ch02-01-guessing-game-tutorial.md)
+- [Guessing Game Tutorial](ch02-00-guessing-game-tutorial.md)
 
-- [Understanding Ownership](ch03-01-understanding-ownership.md)
+- [Understanding Ownership](ch03-00-understanding-ownership.md)
-    - [Ownership](ch03-02-ownership.md)
+    - [Ownership](ch03-01-ownership.md)
-    - [References & Borrowing](ch03-03-references-and-borrowing.md)
+    - [References & Borrowing](ch03-02-references-and-borrowing.md)
-    - [Slices](ch03-04-slices.md)
+    - [Slices](ch03-03-slices.md)
 
-- [Up and Running](ch04-01-up-and-running.md)
+- [Up and Running](ch04-00-up-and-running.md)
-    - [Anatomy of a Rust Program](ch04-02-anatomy-of-a-rust-program.md)
+    - [Anatomy of a Rust Program](ch04-01-anatomy-of-a-rust-program.md)
-    - [Variable Bindings in Detail](ch04-03-variable-bindings-in-detail.md)
+    - [Variable Bindings in Detail](ch04-02-variable-bindings-in-detail.md)
-    - [Data Types in Rust](ch04-04-data-types-in-rust.md)
+    - [Data Types in Rust](ch04-03-data-types-in-rust.md)
-    - [How Functions Work](ch04-05-how-functions-work.md)
+    - [How Functions Work](ch04-04-how-functions-work.md)
-    - [Comments](ch04-06-comments.md)
+    - [Comments](ch04-05-comments.md)
-    - [Control Flow](ch04-07-control-flow.md)
+    - [Control Flow](ch04-06-control-flow.md)
 
-- [Structs](ch05-01-structs.md)
+- [Structs](ch05-00-structs.md)
-    - [Method Syntax](ch05-02-method-syntax.md)
+    - [Method Syntax](ch05-01-method-syntax.md)
-    - [Generics](ch05-03-generics.md)
+    - [Generics](ch05-02-generics.md)
 
-- [Enums](ch06-01-enums.md)
+- [Enums](ch06-00-enums.md)
-    - [Option](ch06-02-option.md)
+    - [Option](ch06-01-option.md)
-    - [Match](ch06-03-match.md)
+    - [Match](ch06-02-match.md)
-    - [Patterns](ch06-04-patterns.md)
+    - [Patterns](ch06-03-patterns.md)
-    - [if let](ch06-05-if-let.md)
+    - [if let](ch06-04-if-let.md)