Add docs on incrementally adopting Next.js.

[leerob]

Building off the "Migrating to Next.js" section started, this doc provides information on using rewrites, redirects, and micro-frontends to incrementally adopt Next.js in your codebase.

[ijjk]

Stats from current PR

Default Server Mode (Increase detected ⚠️)

General

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
buildDuration	13.8s	13.6s	-142ms
nodeModulesSize	85 MB	85 MB	✓

Page Load Tests Overall increase ✓

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.531	2.466	-0.06
/ avg req/sec	987.57	1013.75	+26.18
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.386	1.391	⚠️ +0.01
/error-in-render avg req/sec	1804.1	1797.08	⚠️ -7.02

Client Bundles (main, webpack, commons)

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
677f882d2ed8..776c.js gzip	12.7 kB	12.7 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-95c0df2..6c91.js gzip	6.53 kB	6.53 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.9 kB	58.9 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
677f882d2ed8..dule.js gzip	8.26 kB	8.26 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-8ead8dd..dule.js gzip	5.64 kB	5.64 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.6 kB	53.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_app-3b0cf13..85f8.js gzip	1.28 kB	1.28 kB	✓
_error-6f635..c393.js gzip	3.44 kB	3.44 kB	✓
hooks-d4ffc3..9e0f.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-d9a15a0..130a.js gzip	1.57 kB	1.57 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.98 kB	7.98 kB	✓

Client Pages Modern

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-f2fcc..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-973cce3..dule.js gzip	1.51 kB	1.51 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.6 kB	5.6 kB	✓

Client Build Manifests

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_buildManifest.js gzip	321 B	321 B	✓
_buildManife..dule.js gzip	330 B	330 B	✓
Overall change	651 B	651 B	✓

Rendered Page Sizes

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
index.html gzip	968 B	968 B	✓
link.html gzip	974 B	974 B	✓
withRouter.html gzip	961 B	961 B	✓
Overall change	2.9 kB	2.9 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
buildDuration	15.7s	15.7s	-79ms
nodeModulesSize	85 MB	85 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
677f882d2ed8..776c.js gzip	12.7 kB	12.7 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-95c0df2..6c91.js gzip	6.53 kB	6.53 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.9 kB	58.9 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
677f882d2ed8..dule.js gzip	8.26 kB	8.26 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-8ead8dd..dule.js gzip	5.64 kB	5.64 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.6 kB	53.6 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_app-3b0cf13..85f8.js gzip	1.28 kB	1.28 kB	✓
_error-6f635..c393.js gzip	3.44 kB	3.44 kB	✓
hooks-d4ffc3..9e0f.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-d9a15a0..130a.js gzip	1.57 kB	1.57 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.98 kB	7.98 kB	✓

Client Pages Modern

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-f2fcc..dule.js gzip	2.29 kB	2.29 kB	✓
hooks-cbf13f..dule.js gzip	387 B	387 B	✓
index-b9a643..dule.js gzip	226 B	226 B	✓
link-973cce3..dule.js gzip	1.51 kB	1.51 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.6 kB	5.6 kB	✓

Client Build Manifests

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_buildManifest.js gzip	321 B	321 B	✓
_buildManife..dule.js gzip	330 B	330 B	✓
Overall change	651 B	651 B	✓

Serverless bundles

	vercel/next.js canary	leerob/next.js incremental-adoption	Change
_error.js	918 kB	918 kB	✓
404.html	4.61 kB	4.61 kB	✓
hooks.html	3.86 kB	3.86 kB	✓
index.js	918 kB	918 kB	✓
link.js	976 kB	976 kB	✓
routerDirect.js	969 kB	969 kB	✓
withRouter.js	969 kB	969 kB	✓
Overall change	4.76 MB	4.76 MB	✓

Commit: f85d002