Updated create-next-app docs to include pnpm usage

[styfle]

This PR updates the docs and examples for create-next-app to include pnpm usage.

The following script was used to update every example README:

const fs = require('fs')
const examples = fs.readdirSync('./examples')

for (let example of examples) {
    const filename = `./examples/${example}/README.md`
    const markdown = fs.readFileSync(filename, 'utf8')
    const regex = new RegExp(`^yarn create next-app --example (.*)$`, 'gm')
    const output = markdown.replace(regex, (yarn, group) => {
        const pnpm = `pnpm create next-app -- --example ${group}`
        return `${yarn}\n# or\n${pnpm}`
    })
    fs.writeFileSync(filename, output)
}

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
buildDuration	15.5s	15.1s	-343ms
buildDurationCached	6.1s	6.1s	⚠️ +38ms
nodeModulesSize	467 MB	467 MB	✓

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.947	2.947	✓
/ avg req/sec	848.36	848.46	+0.1
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.194	1.182	-0.01
/error-in-render avg req/sec	2093.6	2115.5	+21.9

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
925.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42 kB	42 kB	✓
main-HASH.js gzip	28.4 kB	28.4 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	72.1 kB	72.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
_app-HASH.js gzip	1.36 kB	1.36 kB	✓
_error-HASH.js gzip	192 B	192 B	✓
amp-HASH.js gzip	309 B	309 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	3.05 kB	3.05 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	920 B	920 B	✓
image-HASH.js gzip	5.48 kB	5.48 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	2.26 kB	2.26 kB	✓
routerDirect..HASH.js gzip	320 B	320 B	✓
script-HASH.js gzip	387 B	387 B	✓
withRouter-HASH.js gzip	319 B	319 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	15.7 kB	15.7 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
index.html gzip	532 B	532 B	✓
link.html gzip	545 B	545 B	✓
withRouter.html gzip	526 B	526 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
buildDuration	18.9s	19.1s	⚠️ +223ms
buildDurationCached	6s	6.1s	⚠️ +27ms
nodeModulesSize	467 MB	467 MB	✓

Page Load Tests Overall increase ✓

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.914	3.01	⚠️ +0.1
/ avg req/sec	858.07	830.65	⚠️ -27.42
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.225	1.198	-0.03
/error-in-render avg req/sec	2040.48	2086.84	+46.36

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
925.HASH.js gzip	178 B	178 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.8 kB	28.8 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	72.7 kB	72.7 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	179 B	179 B	✓
amp-HASH.js gzip	313 B	313 B	✓
css-HASH.js gzip	324 B	324 B	✓
dynamic-HASH.js gzip	3.04 kB	3.04 kB	✓
head-HASH.js gzip	351 B	351 B	✓
hooks-HASH.js gzip	921 B	921 B	✓
image-HASH.js gzip	5.59 kB	5.59 kB	✓
index-HASH.js gzip	261 B	261 B	✓
link-HASH.js gzip	2.33 kB	2.33 kB	✓
routerDirect..HASH.js gzip	322 B	322 B	✓
script-HASH.js gzip	388 B	388 B	✓
withRouter-HASH.js gzip	317 B	317 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	15.8 kB	15.8 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js updated-create-next-app-with-pnpm-readme	Change
index.html gzip	529 B	529 B	✓
link.html gzip	544 B	544 B	✓
withRouter.html gzip	525 B	525 B	✓
Overall change	1.6 kB	1.6 kB	✓

Commit: 836edfe