docs: correct stale routing and url references

Document that the CLI mounts the content directory at the URL root and
that the app uses History API routing, not the old /content hash scheme.
Replace the removed <Article> iconPath prop with the actual upward
assets/ lookup, and fix the deck_preview preview_url example.

src/components/Article.tsx

@@ -109,7 +109,7 @@ export const Article: Component<ArticleProps & ParentProps> = (props) => {
 
       e.preventDefault();
       // Resolve relative hrefs against the current page path so that
-      // e.g. "blah.md" on /content/page.md becomes /content/blah
+      // e.g. "blah.md" on /rules/page becomes /rules/blah
       const resolved = new URL(href, window.location.origin + window.location.pathname);
       let pathname = resolved.pathname;
       // Strip .md extension so refreshes hit the SPA fallback, not the raw file