docs(babelrc): precise usage without configuring plugins

[vvo]

Before this commit, it was unclear if we could add plugins to the babelrc plugins entry directly.

PS: I guess we could also document that all plugins/presets be added in a single way instead of having two ways of doing it.

[ijjk]

Stats from current PR

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General

	vercel/next.js canary	vvo/next.js patch-1	Change
buildDuration	13s	13.1s	⚠️ +165ms
nodeModulesSize	55.9 MB	55.9 MB	✓

Page Load Tests Overall increase ✓

	vercel/next.js canary	vvo/next.js patch-1	Change
/ failed reqs	0	0	✓
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Client Pages Modern

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Rendered Page Sizes

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---

Serverless Mode

General

	vercel/next.js canary	vvo/next.js patch-1	Change
buildDuration	14.9s	15s	⚠️ +45ms
nodeModulesSize	55.9 MB	55.9 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vvo/next.js patch-1	Change
677f882d2ed8..9ff9.js gzip	10.3 kB	10.3 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-e1c4209..e2c9.js gzip	7.32 kB	7.32 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
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Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	vvo/next.js patch-1	Change
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framework.HA..dule.js gzip	39 kB	39 kB	✓
main-9077898..dule.js gzip	6.38 kB	6.38 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
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polyfills-4b..e242.js gzip	31 kB	31 kB	✓
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hooks-89731c..c609.js gzip	887 B	887 B	✓
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routerDirect..924c.js gzip	284 B	284 B	✓
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Client Pages Modern

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hooks-cbf13f..dule.js gzip	387 B	387 B	✓
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routerDirect..dule.js gzip	284 B	284 B	✓
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Client Build Manifests

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Serverless bundles

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routerDirect.js	1.07 MB	1.07 MB	✓
withRouter.js	1.07 MB	1.07 MB	✓
Overall change	5.28 MB	5.28 MB	✓

Commit: aa72acb