doc (book)[04/14]: "Getting Started"/"First Steps...": add intro blurb

In the "First Steps with Cargo" section, add a short introductory
paragraph informing the reader what to expect from the page.

This provides a transition for the reader who may not yet know what
a package is. Rather than jump right into "To start a new package
with Cargo, ...", we now set the context for the purpose of the
page, and also provide hyperlinks into our Glossary for the terms
'package', 'manifest', and 'crate'.

Author: salewski

src/doc/src/getting-started/first-steps.md

@@ -1,13 +1,18 @@
 ## First Steps with Cargo
 
+This section provides a quick sense for the `cargo` command line tool. We
+demonstrate its ability to generate a new [***package***][def-package] for us,
+its ability to compile the [***crate***][def-crate] within the package, and
+its ability to run the resulting program.
+
 To start a new package with Cargo, use `cargo new`:
 
 ```console
 $ cargo new hello_world
 ```
 
-Cargo defaults to `--bin` to make a binary program. To make a library, we'd
-pass `--lib`.
+Cargo defaults to `--bin` to make a binary program. To make a library, we
+would pass `--lib`, instead.
 
 Let’s check out what Cargo has generated for us:
 
@@ -34,8 +39,8 @@ edition = "2018"
 [dependencies]
 ```
 
-This is called a **manifest**, and it contains all of the metadata that Cargo
-needs to compile your package.
+This is called a [***manifest***][def-manifest], and it contains all of the
+metadata that Cargo needs to compile your package.
 
 Here’s what’s in `src/main.rs`:
 
@@ -45,7 +50,8 @@ fn main() {
 }
 ```
 
-Cargo generated a “hello world” for us. Let’s compile it:
+Cargo generated a “hello world” program for us, otherwise known as a
+[***binary crate***][def-crate]. Let’s compile it:
 
 ```console
 $ cargo build
@@ -71,3 +77,7 @@ Hello, world!
 ### Going further
 
 For more details on using Cargo, check out the [Cargo Guide](../guide/index.md)
+
+[def-crate]:     ../appendix/glossary.md#crate     '"crate" (glossary entry)'
+[def-manifest]:  ../appendix/glossary.md#manifest  '"manifest" (glossary entry)'
+[def-package]:   ../appendix/glossary.md#package   '"package" (glossary entry)'