[markdown] update example CSS/JS and add an example readme (#1970)

Also fix the build

.github/workflows/deploy_pages.yaml

@@ -28,7 +28,7 @@ jobs:
       # Build the markdown playground.
       - run: dart pub get
         working-directory: pkgs/markdown
-      - run: dart run build_runner build -o example:build --release
+      - run: dart run build_runner build -o example:build --release --delete-conflicting-outputs --verbose
         working-directory: pkgs/markdown
 
       # Create the _site directory.

pkgs/markdown/build.yaml

@@ -2,9 +2,6 @@
 targets:
   $default:
     builders:
-      build_web_compilers|dart2js_archive_extractor:
-        options:
-          filter_outputs: false
       build_web_compilers|entrypoint:
         generate_for:
           - example/**.dart

pkgs/markdown/example/README.md

@@ -0,0 +1,13 @@
+Commands must be executed from the `/markdown` directory.
+
+Run locally with JavaScript development compiler:
+
+```console
+dart run build_runner serve example
+```
+
+Build production JS and WebAssembly:
+
+```console
+dart run build_runner build -o example:build --release
+```

pkgs/markdown/example/index.html

@@ -4,10 +4,10 @@
     <link rel="stylesheet" href="style.css">
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Roboto+Mono|Roboto">
     <link rel="stylesheet" href="https://fonts.googleapis.com/icon?family=Material+Icons+Extended">
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.5.1/styles/default.min.css" integrity="sha512-hasIneQUHlh06VNBe7f6ZcHmeRTLIaQWFd43YriJ0UND19bvYRauxthDg8E4eVNPm9bRUhr5JGeqH7FRFXQu5g==" crossorigin="anonymous" referrerpolicy="no-referrer" />
-    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/github-markdown-css/5.1.0/github-markdown-light.min.css" integrity="sha512-zb2pp+R+czM7GAemdSUQt6jFmr3qCo6ikvBgVU6F5GvwEDR0C2sefFiPEJ9QUpmAKdD5EqDUdNRtbOYnbF/eyQ==" crossorigin="anonymous" referrerpolicy="no-referrer" />
-    <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.5.1/highlight.min.js" integrity="sha512-yUUc0qWm2rhM7X0EFe82LNnv2moqArj5nro/w1bi05A09hRVeIZbN6jlMoyu0+4I/Bu4Ck/85JQIU82T82M28w==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
-    <script defer src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.5.1/languages/dart.min.js" integrity="sha512-14QR6tzX5xTNeMJKXzSK+xCquDvtNEr1jM5NlKy/149BBY50Kv70qqxHtzo6zClbtc1gIG7G0CGWXuMgPIMt0g==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/styles/default.min.css" integrity="sha512-hasIneQUHlh06VNBe7f6ZcHmeRTLIaQWFd43YriJ0UND19bvYRauxthDg8E4eVNPm9bRUhr5JGeqH7FRFXQu5g==" crossorigin="anonymous" referrerpolicy="no-referrer" />
+    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/github-markdown-css/5.8.1/github-markdown-light.min.css" integrity="sha512-X175XRJAO6PHAUi8AA7GP8uUF5Wiv+w9bOi64i02CHKDQBsO1yy0jLSKaUKg/NhRCDYBmOLQCfKaTaXiyZlLrw==" crossorigin="anonymous" referrerpolicy="no-referrer" />
+    <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/highlight.min.js" integrity="sha512-EBLzUL8XLl+va/zAsmXwS7Z2B1F9HUHkZwyS/VKwh3S7T/U0nF4BaU29EP/ZSf6zgiIxYAnKLu6bJ8dqpmX5uw==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
+    <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.11.1/languages/dart.min.js" integrity="sha512-4NIdIujw2gXoLU7x+versij0q7JzrtjDETxeDaBIb2gM7EctoudrUUtWm+aTx9ODExJTTCLzq06Gjs642x/DwA==" crossorigin="anonymous" referrerpolicy="no-referrer"></script>
     <script defer src="app.dart.js"></script>
     <title>Dart Markdown Live Editor</title>
     <style>