feat(content): parse document preemble

Show title stored in preemble of markdown file if available.

src/main.zig

@@ -6,7 +6,9 @@ pub fn main() !void {
 
     const allocator = gpa.allocator();
 
-    var app: App = .init(.{});
+    var app: App = .init(.{
+        .document = .init(@embedFile("blog")),
+    });
     var renderer = zterm.Renderer.Buffered.init(allocator);
     defer renderer.deinit();
 
@@ -18,6 +20,8 @@ pub fn main() !void {
         },
     }, .{});
     defer container.deinit();
+    var content_container: App.Container = undefined;
+    defer content_container.deinit();
 
     // header with navigation buttons and content's title
     {
@@ -60,7 +64,7 @@ pub fn main() !void {
     // main actual tui_website page content
     {
         var content: Content = .init(allocator);
-        const content_container: App.Container = try .init(allocator, .{}, content.element());
+        content_container = try .init(allocator, .{}, content.element());
 
         var scrollable: App.Scrollable = .init(content_container, .enabled(.green));
         // intermediate container for *padding* containing the scrollable `Content`