feat: add conditional admonition component

sources/academy/webscraping/scraping_basics_javascript2/index.md

@@ -9,11 +9,18 @@ unlisted: true
 ---
 
 import DocCardList from '@theme/DocCardList';
+import UrlConditional from '@site/src/components/UrlConditional';
 
 **Learn how to use JavaScript to extract information from websites in this practical course, starting from the absolute basics.**
 
 ---
 
+<UrlConditional fragment="old-js-course">
+:::tip
+You're seeing this because your URL includes `#old-js-course`.
+:::
+</UrlConditional>
+
 In this course we'll use JavaScript to create an application for watching prices. It'll be able to scrape all product pages of an e-commerce website and record prices. Data from several runs of such program would be useful for seeing trends in price changes, detecting discounts, etc.
 
 ![E-commerce listing on the left, JSON with data on the right](./images/scraping.webp)

src/components/UrlConditional.jsx

@@ -0,0 +1,7 @@
+import { useLocation } from '@docusaurus/router';
+
+export default function UrlConditional({fragment, children}) {
+  const location = useLocation();
+  const shouldShow = location.hash === `#${fragment}`;
+  return shouldShow ? <>{children}</> : null;
+}