Add sample blog posts and fix rendering

- Add hello-world.md and second-post.md sample posts
- Fix index page: use set:html for links, add charset, import global.css
- Fix post page: add charset meta tag
- Fix remark plugin: use className array, handle leaf directives
- Update CSS: support left/right margin note classes

apps/blog/src/styles/global.css

@@ -4,17 +4,23 @@ body {
   padding: 1rem;
 }
 
-.aside {
+.left, .right {
   display: block;
   font-size: 0.9rem;
 }
 
 @media (min-width: 63rem) {
-  .aside {
+  .left, .right {
     display: inline;
-    float: right;
     position: relative;
     width: 10rem;
+  }
+  .left {
+    float: left;
+    margin-left: -12rem;
+  }
+  .right {
+    float: right;
     margin-right: -12rem;
   }
 }