add OsuMarkdownContainer

All of the markdown file in osu-wiki have YAML frontmatter. This YAML is
parsed as common markdown paragraph. So we add `UseYamlFrontMatter()` in
markdown pipeline builder to parse YAML as `YamlFrontMatterBlock`.

osu.Game/Graphics/Containers/Markdown/OsuMarkdownContainer.cs

@@ -0,0 +1,35 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using Markdig;
+using Markdig.Extensions.AutoIdentifiers;
+using Markdig.Extensions.Yaml;
+using Markdig.Syntax;
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics.Containers.Markdown;
+
+namespace osu.Game.Graphics.Containers.Markdown
+{
+    public class OsuMarkdownContainer : MarkdownContainer
+    {
+        protected override void AddMarkdownComponent(IMarkdownObject markdownObject, FillFlowContainer container, int level)
+        {
+            switch (markdownObject)
+            {
+                case YamlFrontMatterBlock _:
+                    // Don't parse YAML Frontmatter
+                    break;
+
+                default:
+                    base.AddMarkdownComponent(markdownObject, container, level);
+                    break;
+            }
+        }
+
+        protected override MarkdownPipeline CreateBuilder()
+            => new MarkdownPipelineBuilder().UseAutoIdentifiers(AutoIdentifierOptions.GitHub)
+                                            .UseEmojiAndSmiley()
+                                            .UseYamlFrontMatter()
+                                            .UseAdvancedExtensions().Build();
+    }
+}