docs: explain swizzling DocCard

demo/docs/customization/doccard.mdx

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+---
+id: doccard
+hide_title: true
+sidebar_label: DocCard Markdown
+title: DocCard Markdown
+description: Swizzle the DocCard component to render Markdown in card descriptions.
+---
+
+## Overview
+
+The default DocCard component used by Docusaurus displays the `description` field
+as plain text. Any Markdown inside the description is escaped rather than
+rendered. To support Markdown, you can swizzle the component and update it to use
+`@theme/Markdown`.
+
+## Swizzling the component
+
+Run the swizzle command from your site directory and choose the **eject** option
+when prompted:
+
+```bash
+npx docusaurus swizzle @docusaurus/theme-classic DocCard
+```
+
+This will create `src/theme/DocCard/index.tsx` in your project. Replace its
+contents with the following:
+
+```tsx title="src/theme/DocCard/index.tsx"
+import React, { type ReactNode } from "react";
+
+import isInternalUrl from "@docusaurus/isInternalUrl";
+import Link from "@docusaurus/Link";
+import type {
+  PropSidebarItemCategory,
+  PropSidebarItemLink,
+} from "@docusaurus/plugin-content-docs";
+import {
+  useDocById,
+  findFirstSidebarItemLink,
+} from "@docusaurus/plugin-content-docs/lib/client/docsUtils";
+import { usePluralForm } from "@docusaurus/theme-common";
+import { translate } from "@docusaurus/Translate";
+import type { Props } from "@theme/DocCard";
+import Heading from "@theme/Heading";
+import Markdown from "@theme/Markdown";
+import clsx from "clsx";
+
+import styles from "./styles.module.css";
+
+function useCategoryItemsPlural() {
+  const { selectMessage } = usePluralForm();
+  return (count: number) =>
+    selectMessage(
+      count,
+      translate(
+        {
+          message: "1 item|{count} items",
+          id: "theme.docs.DocCard.categoryDescription.plurals",
+          description:
+            "The default description for a category card in the generated index about how many items this category includes",
+        },
+        { count }
+      )
+    );
+}
+
+function CardContainer({
+  className,
+  href,
+  children,
+}: {
+  className?: string;
+  href: string;
+  children: ReactNode;
+}): ReactNode {
+  return (
+    <Link
+      href={href}
+      className={clsx("card padding--lg", styles.cardContainer, className)}
+    >
+      {children}
+    </Link>
+  );
+}
+
+function CardLayout({
+  className,
+  href,
+  icon,
+  title,
+  description,
+}: {
+  className?: string;
+  href: string;
+  icon: ReactNode;
+  title: string;
+  description?: string;
+}): ReactNode {
+  return (
+    <CardContainer href={href} className={className}>
+      <Heading
+        as="h2"
+        className={clsx("text--truncate", styles.cardTitle)}
+        title={title}
+      >
+        {icon} {title}
+      </Heading>
+      {description && (
+        <Markdown
+          className={clsx("text--truncate", styles.cardDescription)}
+          title={description}
+        >
+          {description}
+        </Markdown>
+      )}
+    </CardContainer>
+  );
+}
+
+function CardCategory({ item }: { item: PropSidebarItemCategory }): ReactNode {
+  const href = findFirstSidebarItemLink(item);
+  const categoryItemsPlural = useCategoryItemsPlural();
+
+  // Unexpected: categories that don't have a link have been filtered upfront
+  if (!href) {
+    return null;
+  }
+
+  return (
+    <CardLayout
+      className={item.className}
+      href={href}
+      icon="🗃️"
+      title={item.label}
+      description={item.description ?? categoryItemsPlural(item.items.length)}
+    />
+  );
+}
+
+function CardLink({ item }: { item: PropSidebarItemLink }): ReactNode {
+  const icon = isInternalUrl(item.href) ? "📄️" : "🔗";
+  const doc = useDocById(item.docId ?? undefined);
+  return (
+    <CardLayout
+      className={item.className}
+      href={item.href}
+      icon={icon}
+      title={item.label}
+      description={item.description ?? doc?.description}
+    />
+  );
+}
+
+export default function DocCard({ item }: Props): ReactNode {
+  switch (item.type) {
+    case "link":
+      return <CardLink item={item} />;
+    case "category":
+      return <CardCategory item={item} />;
+    default:
+      throw new Error(`unknown item type ${JSON.stringify(item)}`);
+  }
+}
+```
+