…alexcrichton

doc (book): add "Getting Started" subsection: "Essential Terminology"

# Reworked PR (2020-11-21)

Fixes #8854

In The Cargo Book, beef-up the glossary, and provide hyperlinks to the definitions of key terms through both the "Getting Started" and "Cargo Guide" chapters.

In addition to the links, some transitional text (two sentences) has been added to the beginning of "First Steps with Cargo", and somewhat more to "Why Cargo Exists" in the guide. The intent is cater to readers jumping into this book cold, or perhaps coming from the "Hello, Cargo!" section at the beginning of The Book. The changes assume a knowledgeable but skeptical user, so take pains to justify the reader's attention. A developer that knows very little about Rust and nothing about Cargo and can follow the text and map Cargo-specific concepts to analogues in other language ecosystems.

Within the glossary many terms are touched-up, as described in the commit message for the first patch in the series. Intra-glossary links are added between all terms.

In the "Getting Started" and "Cargo Guide" chapters, terms are linked to their glossary entries throughout, for the first mention of a term within a given *section*. The first time a term is seen in the *chapter*, the markdown syntax for italics is used. In "First Steps with Cargo", both italics and bold markdown are used, in keeping with the existing styling.

# Orig PR (2020-11-12) -- obsolete

~~Insert short terminology cheatsheet page to explain these terms:~~

   * ~~package~~
   * ~~crate~~
      - ~~library crate~~
      - ~~binary crate~~
   * ~~module~~
   * ~~workspace~~

~~Important in these definitions is the fact that a single package is defined by
a single 'Cargo.toml' file, and that a non-workspace Cargo.toml package
contains one or more crates.~~

~~The last two terms in the list are just glossed over, as it is more important
that the user know that the terms exist, and that there are both lower-level
(module) and higher-level (workspace) concepts than the organizational units
(a package and its crates) that feature most prominently in basic cargo
builds.~~

~~With the addition of this new "Essential Terminology" page, the structure at
the beginning of The Cargo Book is:~~

    Introduction
    1. Getting Started
        1.1. Essential Terminology      <= new
        1.2. Installation
        1.3. First Steps with Cargo
    2. Cargo Guide
    ...

~~The terms in the above list are all well defined elsewhere in the Rust and
Cargo documentation, but a new user encountering them for the first time
should not need to hunt them down.~~

~~Since the opening sections of the Cargo book are among the first documents a
new user is likely to encounter, we are providing a little cheatsheet of the
essentials at this stage. Defining the above terms gives the reader a good
foothold to map concepts they may be familiar with from other languages onto
their Rust counterparts (roughly speaking).~~

~~Closes #8854~~