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

build.zig

@@ -141,7 +141,7 @@ pub fn build(b: *std.Build) void {
     const test_includes_exe = b.addExecutable(.{
         .name = "test-includes",
         .root_module = b.createModule(.{
-            .root_source_file = b.path("examples/test_includes.zig"),
+            .root_source_file = b.path("src/tests/test_includes.zig"),
             .target = target,
             .optimize = optimize,
             .imports = &.{