Move frontmatter intro and intro example ahead of grammar

Many of our existing chapters start directly with the grammar.  We're
shifting, however, toward leading with the introduction and with the
introductory example.  Let's do that for the frontmatter chapter.

Author: traviscross

src/frontmatter.md

@@ -1,6 +1,20 @@
 r[frontmatter]
 # Frontmatter
 
+r[frontmatter.intro]
+Frontmatter is an optional section of metadata whose syntax allows external tools to read it without parsing Rust.
+
+> [!EXAMPLE]
+> <!-- ignore: test runner doesn't support frontmatter -->
+> ```rust,ignore
+> #!/usr/bin/env cargo
+> --- cargo
+> package.edition = 2024
+> ---
+>
+> fn main() {}
+> ```
+
 r[frontmatter.syntax]
 ```grammar,lexer
 @root FRONTMATTER ->
@@ -34,20 +48,6 @@ MAYBE_INFOSTRING_OR_WS ->
 INFOSTRING -> (XID_Start | `_`) ( XID_Continue | `-` | `.` )*
 ```
 
-r[frontmatter.intro]
-Frontmatter is an optional section of metadata whose syntax allows external tools to read it without parsing Rust.
-
-> [!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].