Added next-sitemap example (#15997)

Co-authored-by: Luis Alvarez <luis@vercel.com>

examples/with-next-sitemap/README.md

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+# next-sitemap example
+
+This example uses [`next-sitemap`](https://github.com/iamvishnusankar/next-sitemap) to generate a sitemap file for all pages (including all pre-rendered/static pages).
+
+`next-sitemap` allows the generation of sitemaps along with `robots.txt` and provides the feature to split large sitemaps into multiple files. Checkout the [`next-sitemap` documentation](https://github.com/iamvishnusankar/next-sitemap) to learn more.
+
+## Deploy your own
+
+Deploy the example using [Vercel](https://vercel.com):
+
+[![Deploy with Vercel](https://vercel.com/button)](https://vercel.com/import/project?template=https://github.com/vercel/next.js/tree/canary/examples/with-next-sitemap)
+
+## How to use
+
+Execute [`create-next-app`](https://github.com/vercel/next.js/tree/canary/packages/create-next-app) with [npm](https://docs.npmjs.com/cli/init) or [Yarn](https://yarnpkg.com/lang/en/docs/cli/create/) to bootstrap the example:
+
+```bash
+npx create-next-app --example with-next-sitemap with-next-sitemap-app
+# or
+yarn create next-app --example with-next-sitemap with-next-sitemap-app
+```
+
+Deploy it to the cloud with [Vercel](https://vercel.com/import?filter=next.js&utm_source=github&utm_medium=readme&utm_campaign=next-example) ([Documentation](https://nextjs.org/docs/deployment)).

examples/with-next-sitemap/next-env.d.ts

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+/// <reference types="next" />
+/// <reference types="next/types/global" />

examples/with-next-sitemap/next-sitemap.js

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+module.exports = {
+  siteUrl: 'https://example.com',
+  generateRobotsTxt: true,
+  // optional
+  robotsTxtOptions: {
+    additionalSitemaps: [
+      'https://example.com/my-custom-sitemap-1.xml',
+      'https://example.com/my-custom-sitemap-2.xml',
+      'https://example.com/my-custom-sitemap-3.xml',
+    ],
+  },
+}

examples/with-next-sitemap/package.json

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+{
+  "name": "with-next-sitemap",
+  "version": "1.0.0",
+  "license": "MIT",
+  "scripts": {
+    "dev": "next",
+    "build": "next build",
+    "start": "next start",
+    "postbuild": "next-sitemap"
+  },
+  "dependencies": {
+    "next": "latest",
+    "react": "^16.13.1",
+    "react-dom": "^16.13.1"
+  },
+  "devDependencies": {
+    "@types/node": "14.6.0",
+    "@types/react": "^16.9.45",
+    "next-sitemap": "latest"
+  }
+}

examples/with-next-sitemap/pages/[dynamic].tsx

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+import { GetStaticPaths, GetStaticProps } from 'next'
+import { useRouter } from 'next/router'
+
+const DynamicPage = () => {
+  const { query } = useRouter()
+
+  return (
+    <div>
+      <h1>Dynamic Page</h1>
+      <h2>Query: {query.dynamic}</h2>
+    </div>
+  )
+}
+
+export const getStaticProps: GetStaticProps = async () => {
+  return {
+    props: {
+      dynamic: 'hello',
+    },
+  }
+}
+
+export const getStaticPaths: GetStaticPaths = async () => {
+  return {
+    paths: [...Array(10000)].map((_, index) => ({
+      params: {
+        dynamic: `page-${index}`,
+      },
+    })),
+    fallback: false,
+  }
+}
+
+export default DynamicPage

examples/with-next-sitemap/pages/index.tsx

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+import Link from 'next/link'
+
+const HelloWorld = () => (
+  <div>
+    <h1>Hello World Page</h1>
+    <ol>
+      <li>
+        <Link href="/[dynamic]" as="/page-1">
+          <a>Link to dynamic page 1</a>
+        </Link>
+      </li>
+      <li>
+        <Link href="/[dynamic]" as="/page-2">
+          <a>Link to dynamic page 2</a>
+        </Link>
+      </li>
+      <li>
+        <Link href="/[dynamic]" as="/page-3">
+          <a>Link to dynamic page 3</a>
+        </Link>
+      </li>
+      <li>
+        <Link href="/[dynamic]" as="/page-4">
+          <a>Link to dynamic page 4</a>
+        </Link>
+      </li>
+      <li>
+        <Link href="/[dynamic]" as="/page-5">
+          <a>Link to dynamic page 5</a>
+        </Link>
+      </li>
+    </ol>
+  </div>
+)
+
+export default HelloWorld

examples/with-next-sitemap/tsconfig.json

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+{
+  "compilerOptions": {
+    "target": "es5",
+    "lib": ["dom", "dom.iterable", "esnext"],
+    "allowJs": true,
+    "skipLibCheck": true,
+    "strict": false,
+    "forceConsistentCasingInFileNames": true,
+    "noEmit": true,
+    "esModuleInterop": true,
+    "module": "esnext",
+    "moduleResolution": "node",
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "jsx": "preserve"
+  },
+  "include": ["next-env.d.ts", "**/*.ts", "**/*.tsx"],
+  "exclude": ["node_modules"]
+}