engage: fix build with mdbook 0.5

Update engage documentation build for mdbook 0.5 compatibility.

Changes:
- Remove deprecated 'multilingual' field from book.toml
- Update git-repository-icon from 'fa-git-square' to 'fab-square-git'

The 'multilingual' field was removed in mdbook 0.5 as it was never used.
The FontAwesome icon name changed from 'fa-git-square' (FA 4) to
'fab-square-git' (FA 5 brand icon) in the version bundled with mdbook 0.5.

pkgs/by-name/en/engage/mdbook-0.5-support.patch

@@ -0,0 +1,16 @@
+--- a/book.toml
++++ b/book.toml
+@@ -1,6 +1,5 @@
+ [book]
+ language = "en"
+-multilingual = false
+ src = "book"
+ title = "Engage"
+
+@@ -8,5 +7,5 @@ build-dir = "public"
+
+ [output.html]
+-git-repository-icon = "fa-git-square"
++git-repository-icon = "fab-square-git"
+ git-repository-url = "https://gitlab.computer.surgery/charles/engage"
+

pkgs/by-name/en/engage/package.nix

@@ -28,6 +28,11 @@ rustPlatform.buildRustPackage (finalAttrs: {
     hash = "sha256-n7ypFJBYT712Uzh1NnWWSOIpEDKR0e6sQxbiIN6pZgo=";
   };
 
+  patches = [
+    # Support mdbook 0.5.x - remove deprecated multilingual field
+    ./mdbook-0.5-support.patch
+  ];
+
   cargoHash = "sha256-UTIxxPBtxzsZilxriAT8ksl2ovoDzIhB+8f+b2cGN3k=";
 
   nativeBuildInputs = [