Add package.json instructions to blog documentation (shuding#2919)

Co-authored-by: Dimitri POSTOLOV <[email protected]>

docs/pages/docs/blog-theme/start.mdx

@@ -32,6 +32,25 @@ npm i next react react-dom nextra nextra-theme-blog
   install `nextra` and `nextra-theme-blog` as the add-ons.
 </Callout>
 
+Add the following scripts in `package.json`:
+
+```json filename="package.json"
+"scripts": {
+  "dev": "next",
+  "build": "next build",
+  "start": "next start"
+},
+```
+
+You can either start the server with, for example if you use npm, `npm run dev`,
+which will run in development mode or `npm run build && npm run start` for
+production mode.
+
+<Callout>
+  If you're not familiar with Next.js, note that development mode is
+  significantly slower since Next.js compiles every page you navigate to.
+</Callout>
+
 ### Add Next.js Config
 
 Create the following `next.config.js` file in your project’s root directory: