feat: add include demo page to showcase include directive

- Add includes/some_partial.pug partial in demo views
- Add pages/include-demo.pug page using include directive
- Add /include-demo route in demo server
- Add link to include demo in about page
- Fix test_includes.zig path in build.zig
- Add test_views for include testing

src/tests/test_includes.zig

@@ -6,6 +6,7 @@ pub fn main() !void {
     defer _ = gpa.deinit();
     const allocator = gpa.allocator();
 
+    // Test: Simple include from test_views
     var engine = pugz.ViewEngine.init(allocator, .{
         .views_dir = "test_views",
     }) catch |err| {
@@ -21,4 +22,14 @@ pub fn main() !void {
     defer allocator.free(html);
 
     std.debug.print("=== Rendered HTML ===\n{s}\n=== End ===\n", .{html});
+
+    // Verify output contains included content
+    if (std.mem.indexOf(u8, html, "Included Partial") != null and
+        std.mem.indexOf(u8, html, "info-box") != null)
+    {
+        std.debug.print("\nSUCCESS: Include directive works correctly!\n", .{});
+    } else {
+        std.debug.print("\nFAILURE: Include content not found!\n", .{});
+        return error.TestFailed;
+    }
 }