[Sistent] fix: add missing Card import in MDX files

The Card component was being used in the MDX examples without being
imported, causing the Gatsby build to fail with "Expected component
Card to be defined". Added the import from @sistent/sistent to all
three files.

Signed-off-by: Abhay Chaurasiya <abhaychaurasiya19@gmail.com>
Signed-off-by: abhay1999 <abhaychaurasiya19@gmail.com>

Author: abhay1999

src/collections/sistent/components/card/code.mdx

@@ -4,6 +4,8 @@ component: card
 description: A card is a contained surface that groups related information and actions about a single subject, making content easy to scan and interact with.
 ---
 
+import { Card, Button } from "@sistent/sistent";
+
 The Card component wraps MUI's Card and works with Sistent's theming out of the box. Here are the most common usage patterns.
 
 <a id="Basic Card">

src/collections/sistent/components/card/guidance.mdx

@@ -4,6 +4,8 @@ component: card
 description: A card is a contained surface that groups related information and actions about a single subject, making content easy to scan and interact with.
 ---
 
+import { Card } from "@sistent/sistent";
+
 Cards work best when they follow a consistent structure and are used intentionally. Here's how to get the most out of them.
 
 <a id="When to Use">

src/collections/sistent/components/card/index.mdx

@@ -6,6 +6,8 @@ component: card
 description: A card is a contained surface that groups related information and actions about a single subject, making content easy to scan and interact with.
 ---
 
+import { Card } from "@sistent/sistent";
+
 Cards are versatile containers that bring together content and actions around a single topic. They give users a clean, scannable way to browse and interact with information — whether that's a product, a profile, a post, or anything in between. Cards can live in grids, lists, or standalone layouts, and they scale well across screen sizes.
 
 <a id="Types">