Add example use of rule in link definition

On the introduction page, where we talk about rule identifiers, let's
add an example use of putting the rule identifier in the link
definition so that we have an easy way of verifying that this
mechanism works as intended.

Author: traviscross

src/introduction.md

@@ -116,7 +116,7 @@ These conventions are documented here.
   See [Notation] for more detail.
 
 r[example.rule.label]
-* Rule identifiers appear before each language rule enclosed in square brackets. These identifiers provide a way to refer to a specific rule in the language. The rule identifier uses periods to separate sections from most general to most specific ([destructors.scope.nesting.function-body] for example). On narrow screens, the rule name will collapse to display `[*]`.
+* Rule identifiers appear before each language rule enclosed in square brackets. These identifiers provide a way to refer to and link to a specific rule in the language ([e.g.][example rule]). The rule identifier uses periods to separate sections from most general to most specific ([destructors.scope.nesting.function-body] for example). On narrow screens, the rule name will collapse to display `[*]`.
 
   The rule name can be clicked to link to that rule.
 
@@ -144,6 +144,7 @@ We also want the reference to be as normative as possible, so if you see anythin
 [_Expression_]: expressions.md
 [cargo book]: ../cargo/index.html
 [cargo reference]: ../cargo/reference/index.html
+[example rule]: example.rule.label
 [expressions chapter]: expressions.html
 [file an issue]: https://github.com/rust-lang/reference/issues
 [lifetime of temporaries]: expressions.html#temporaries