Add remark-toc plugin.

Add plugin for generating table of content automatically for markdown files.

src/components/article/ArticleBody.astro

@@ -45,7 +45,7 @@ const articlePrettyName = article!.id.split("/").pop()!.replace(".md", "");
     flex-wrap: wrap;
   }
   /* reduce indent of TOC list */
-  :global(.article-title + ul) {
+  :global(.article-title ~ ul:nth-of-type(1)) {
     margin: 1em 0 1em 0;
     padding: 0 0 0 20px;
   }