Revise frontmatter intro example

The example under `frontmatter.intro` used an external crate, a
nontrivial script body, and a bare `rust` code block that would
fail CI since the test runner doesn't support frontmatter.  Let's
simplify it to mirror the example in the frontmatter removal section
of `input-format.md`, and let's wrap it in an `EXAMPLE` admonition
consistent with our convention for examples that aren't demonstrating
the behavior of a specific rule.

Author: traviscross

src/frontmatter.md

@@ -37,18 +37,16 @@ INFOSTRING -> (XID_Start | `_`) ( XID_Continue | `-` | `.` )*
 r[frontmatter.intro]
 Frontmatter is an optional section for content intended for external tools without requiring these tools to have full knowledge of the Rust grammar.
 
-```rust
-#!/usr/bin/env cargo
----
-[dependencies]
-fastrand = "2"
----
-
-fn main() {
-    let num = fastrand::i32(..);
-    println!("{num}");
-}
-```
+> [!EXAMPLE]
+> <!-- ignore: test runner doesn't support frontmatter -->
+> ```rust,ignore
+> #!/usr/bin/env cargo
+> --- cargo
+> package.edition = 2024
+> ---
+>
+> fn main() {}
+> ```
 
 r[frontmatter.position]
 Frontmatter may appear at the start of the file (after the optional [byte order mark]) or after a [shebang]. In either case, it may be preceded by [whitespace].