docs: add markdown renderer migration guide for v4 to v5

Adds documentation for migrating custom markdown renderer overrides
from v4 to v5. The window..markdown option is deprecated
in v5, so this guide provides alternative approaches using:

1. marked extensions (recommended)
2. Docsify plugins with custom renderers

Closes #2680

Author: liwenjun-dev

docs/v5-upgrade.md

@@ -117,6 +117,107 @@ View [Theme Classes](themes.md?id=classes) for more details.
 - **Themes**: v5 uses a core theme (with optional add-ons available)
 - **Plugin Names**: `zoom-image` → `zoom`
 
+## Migrating Custom Markdown Renderer Overrides
+
+If you were customizing the Markdown renderer in v4 using the `window.$docsify.markdown` configuration option, you'll need to update your approach for v5.
+
+### v4 Approach (Deprecated)
+
+In v4, you could customize the marked renderer like this:
+
+```javascript
+window.$docsify = {
+  markdown: function (marked, renderer) {
+    marked.setOptions({
+      smartypants: true,
+      renderer: Object.assign(renderer, {
+        paragraph(text) {
+          // Custom paragraph rendering
+          return marked.Renderer.prototype.paragraph.apply(null, [text]);
+        },
+        text(text) {
+          // Custom text rendering
+          return marked.Renderer.prototype.text.apply(null, [text]);
+        },
+      }),
+    });
+    return marked;
+  },
+};
+```
+
+### v5 Approach
+
+In v5, Docsify uses **marked v13+**, which has a different architecture. The `window.$docsify.markdown` option is no longer supported. Instead, use one of these approaches:
+
+#### Option 1: Using marked Extensions (Recommended)
+
+Load your marked extension before Docsify and register it in the configuration:
+
+```html
+<!-- Load your marked extension -->
+<script src="https://cdn.jsdelivr.net/npm/marked-footnote@1.2.0/dist/index.umd.min.js"></script>
+
+<script>
+window.$docsify = {
+  marked: {
+    extensions: [markedFootnote()], // Register the extension
+  },
+};
+</script>
+```
+
+#### Option 2: Using a Docsify Plugin
+
+Create a plugin that customizes the renderer after Docsify initializes:
+
+```javascript
+window.$docsify = {
+  // ... your other config
+};
+
+// Add a custom plugin
+window.$docsify.plugins = [
+  function (hook, vm) {
+    hook.doneEach(function () {
+      // Access the marked instance
+      const marked = vm.config.marked;
+      if (marked) {
+        const { Renderer } = marked;
+
+        // Create a custom renderer
+        const renderer = new Renderer();
+        const originalParagraph = renderer.paragraph.bind(renderer);
+        const originalText = renderer.text.bind(renderer);
+
+        // Override methods
+        renderer.paragraph = function (text) {
+          // Apply your custom transformations here
+          return originalParagraph(text);
+        };
+
+        renderer.text = function (text) {
+          // Apply your custom transformations here
+          return originalText(text);
+        };
+
+        // Update marked options
+        marked.setOptions({ renderer });
+      }
+    });
+  },
+];
+```
+
+#### Key Changes
+
+- ✅ Docsify v5 uses **marked v13+** with a hooks-based architecture
+- ✅ The `window.$docsify.markdown` config option is **deprecated**
+- ✅ Use `window.$docsify.marked` for passing marked options and extensions
+- ✅ Custom renderers should use the **extensions system** or a **Docsify plugin**
+
+For more details on marked extensions, see the [marked documentation](https://marked.js.org/using_advanced#extensions).
+
 ## Additional Notes
 
 - Your configuration in `window.$docsify` stays the same