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

tshepang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,rustyhorde/cargo,tshepang/cargo,nipunn1313/cargo,tshepang/cargo,rust-lang/cargo,nipunn1313/cargo,rustyhorde/cargo,eminence/cargo,rust-lang/cargo,tshepang/cargo,rustyhorde/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,rust-lang/cargo,eminence/cargo