extending the docs to explain how to use embedInCode (#9231)

Extends the PackageDescription documentation to explain how to use the
.embedInCode rule for a resource.

### Motivation:

The changelog originally included some detail, but it's missing from the
documentation, so updating the
API reference for PackageDescription to provide information about how to
use .embedInCode.

### Modifications:

Updated the .embedInCode documentation to show code blocks with an
example of usage to illustrate how to use it.

### Result:

Improved documentation for PackageDescription and the embedInCode
resource rule

---------

Co-authored-by: Bri Peticca <bripeticca@gmail.com>

Author: heckj

Sources/Runtimes/PackageDescription/Resource.swift

@@ -97,7 +97,20 @@ public struct Resource: Sendable {
 
     /// Applies the embed rule to a resource at the given path.
     ///
-    /// You can use the embed rule to embed the contents of the resource into the executable code.
+    /// Use the embed rule to embed the bytes that represent the contents of a resource into executable code.
+    /// For example,  if you embed the file `identifier.txt` that has the contents:
+    /// ```
+    /// Hello Swift
+    /// ```
+    ///
+    /// Package manager generates a PackageResources class with each embedded resource as a static property based on the name of the resource.
+    /// For the example above, the code generated by Package Manager is equivalent to the following source:
+    /// ```
+    /// struct PackageResources {
+    ///     static let identifier_txt: [UInt8] = [72,101,108,108,111,32,83,119,105,102,116,10]
+    /// }
+    /// ```
+    ///
     /// - Parameter path: The path for a resource.
     /// - Returns: A `Resource` instance.
     @available(_PackageDescription, introduced: 5.9)