LibMarkdown: Add render_to_inline_html() to Document

This api is useful when you want to render a markdown document to HTML,
but you want to embed it in a existing html document.

Userland/Libraries/LibMarkdown/Document.h

@@ -15,6 +15,7 @@ namespace Markdown {
 class Document final {
 public:
     String render_to_html() const;
+    String render_to_inline_html() const;
     String render_for_terminal(size_t view_width = 0) const;
 
     static OwnPtr<Document> parse(const StringView&);