Update image optimization docs to include blog details

[styfle]

This PR documents some details that are mentioned in the blog post https://nextjs.org/blog/next-10

[balazsorban44]

What I still don't see answered is if the generated optimized images are saved somewhere for subsequent users, or are they generated on the fly every time for each user?

[styfle]

@balazsorban44 Thats mentioned in the Caching section below, added in PR #18399

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
buildDuration	13.5s	14.2s	⚠️ +671ms
nodeModulesSize	91.3 MB	91.3 MB	✓

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.614	2.689	⚠️ +0.08
/ avg req/sec	956.4	929.8	⚠️ -26.6
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.589	1.603	⚠️ +0.01
/error-in-render avg req/sec	1573.77	1559.56	⚠️ -14.21

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
677f882d2ed8..3aa9.js gzip	11.3 kB	11.3 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-fc2ed98..cd2b.js gzip	7.33 kB	7.33 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.3 kB	58.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
677f882d2ed8..dule.js gzip	7.03 kB	7.03 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-20f9619..dule.js gzip	6.31 kB	6.31 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.1 kB	53.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-41ceea0..6b62.js gzip	1.35 kB	1.35 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.75 kB	7.75 kB	✓

Client Pages Modern

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..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-2d41a89..dule.js gzip	1.31 kB	1.31 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.4 kB	5.4 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_buildManifest.js gzip	323 B	323 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	652 B	652 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
index.html gzip	1 kB	1 kB	✓
link.html gzip	1.01 kB	1.01 kB	✓
withRouter.html gzip	996 B	996 B	✓
Overall change	3.01 kB	3.01 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
buildDuration	15.2s	15.6s	⚠️ +407ms
nodeModulesSize	91.3 MB	91.3 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
677f882d2ed8..3aa9.js gzip	11.3 kB	11.3 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-fc2ed98..cd2b.js gzip	7.33 kB	7.33 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	58.3 kB	58.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
677f882d2ed8..dule.js gzip	7.03 kB	7.03 kB	✓
framework.HA..dule.js gzip	39 kB	39 kB	✓
main-20f9619..dule.js gzip	6.31 kB	6.31 kB	✓
webpack-07c5..dule.js gzip	751 B	751 B	✓
Overall change	53.1 kB	53.1 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_app-9a0b9e1..b37e.js gzip	1.28 kB	1.28 kB	✓
_error-ed1b0..8fbd.js gzip	3.44 kB	3.44 kB	✓
hooks-89731c..c609.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-41ceea0..6b62.js gzip	1.35 kB	1.35 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	7.75 kB	7.75 kB	✓

Client Pages Modern

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_app-75d3a82..dule.js gzip	625 B	625 B	✓
_error-4469a..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-2d41a89..dule.js gzip	1.31 kB	1.31 kB	✓
routerDirect..dule.js gzip	284 B	284 B	✓
withRouter-f..dule.js gzip	282 B	282 B	✓
Overall change	5.4 kB	5.4 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_buildManifest.js gzip	323 B	323 B	✓
_buildManife..dule.js gzip	329 B	329 B	✓
Overall change	652 B	652 B	✓

Serverless bundles

	vercel/next.js canary	vercel/next.js image-optimiazation-no-build-docs	Change
_error.js	1.06 MB	1.06 MB	✓
404.html	4.73 kB	4.73 kB	✓
hooks.html	3.92 kB	3.92 kB	✓
index.js	1.06 MB	1.06 MB	✓
link.js	1.11 MB	1.11 MB	✓
routerDirect.js	1.1 MB	1.1 MB	✓
withRouter.js	1.1 MB	1.1 MB	✓
Overall change	5.43 MB	5.43 MB	✓

Commit: df84808