doc (book)[02/14]: "Introduction": add links to glossary for key terms

No language changes; just make these existing terms link to their
corresponding entries in the glossary:
   - crate
   - package
   - package manager
   - package registry

Author: salewski

src/doc/src/index.md

@@ -2,17 +2,18 @@
 
 ![Cargo Logo](images/Cargo-Logo-Small.png)
 
-Cargo is the [Rust] *package manager*. Cargo downloads your Rust package’s
+Cargo is the [Rust] [*package manager*][def-package-manager]. Cargo downloads your Rust [package][def-package]'s
 dependencies, compiles your packages, makes distributable packages, and uploads them to
-[crates.io], the Rust community’s *package registry*. You can contribute
+[crates.io], the Rust community’s [*package registry*][def-package-registry]. You can contribute
 to this book on [GitHub].
 
 
 ### Sections
 
 **[Getting Started](getting-started/index.md)**
 
-To get started with Cargo, install Cargo (and Rust) and set up your first crate.
+To get started with Cargo, install Cargo (and Rust) and set up your first
+[*crate*][def-crate].
 
 **[Cargo Guide](guide/index.md)**
 
@@ -39,6 +40,10 @@ The commands will let you interact with Cargo using its command-line interface.
 * [Rust documentation website](https://doc.rust-lang.org/) — Links to official
   Rust documentation and tools.
 
+[def-crate]:            ./appendix/glossary.md#crate            '"crate" (glossary entry)'
+[def-package]:          ./appendix/glossary.md#package          '"package" (glossary entry)'
+[def-package-manager]:  ./appendix/glossary.md#package-manager  '"package manager" (glossary entry)'
+[def-package-registry]: ./appendix/glossary.md#package-registry '"package registry" (glossary entry)'
 [rust]: https://www.rust-lang.org/
 [crates.io]: https://crates.io/
 [GitHub]: https://github.com/rust-lang/cargo/tree/master/src/doc