Add "title" as the source for "slug" in cms-sanity example

[btk]

The cms "sanity" has a feature for slugs to be generated from another field.

This change adds the title as source to slug, so in the editor you will have a button like this to generate.

[ijjk]

Stats from current PR

Default Server Mode (Decrease detected ✓)

General

	vercel/next.js canary	btk/next.js patch-1	Change
buildDuration	9.7s	9.7s	⚠️ +27ms
nodeModulesSize	82.4 MB	82.4 MB	✓

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	btk/next.js patch-1	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.376	2.481	⚠️ +0.1
/ avg req/sec	1052.3	1007.59	⚠️ -44.71
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.274	1.266	-0.01
/error-in-render avg req/sec	1961.75	1975.01	+13.26

Client Bundles (main, webpack, commons)

	vercel/next.js canary	btk/next.js patch-1	Change
677f882d2ed8..34b9.js gzip	12.8 kB	12.8 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-f501b4a..c42e.js gzip	6.56 kB	6.56 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	59 kB	59 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	btk/next.js patch-1	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	btk/next.js patch-1	Change
_app-7231d4b..5856.js gzip	1.28 kB	1.28 kB	✓
_error-01375..90b6.js gzip	3.69 kB	3.69 kB	✓
hooks-d4591d..e7c2.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-db223d9..dbd7.js gzip	1.61 kB	1.61 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	8.27 kB	8.27 kB	✓

Client Build Manifests

	vercel/next.js canary	btk/next.js patch-1	Change
_buildManifest.js gzip	322 B	322 B	✓
Overall change	322 B	322 B	✓

Rendered Page Sizes

	vercel/next.js canary	btk/next.js patch-1	Change
index.html gzip	614 B	614 B	✓
link.html gzip	621 B	621 B	✓
withRouter.html gzip	608 B	608 B	✓
Overall change	1.84 kB	1.84 kB	✓

---

Serverless Mode

General

	vercel/next.js canary	btk/next.js patch-1	Change
buildDuration	11.6s	12.3s	⚠️ +689ms
nodeModulesSize	82.4 MB	82.4 MB	✓

Client Bundles (main, webpack, commons)

	vercel/next.js canary	btk/next.js patch-1	Change
677f882d2ed8..34b9.js gzip	12.8 kB	12.8 kB	✓
framework.HASH.js gzip	39 kB	39 kB	✓
main-f501b4a..c42e.js gzip	6.56 kB	6.56 kB	✓
webpack-e067..f178.js gzip	751 B	751 B	✓
Overall change	59 kB	59 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	btk/next.js patch-1	Change
polyfills-4b..e242.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	btk/next.js patch-1	Change
_app-7231d4b..5856.js gzip	1.28 kB	1.28 kB	✓
_error-01375..90b6.js gzip	3.69 kB	3.69 kB	✓
hooks-d4591d..e7c2.js gzip	887 B	887 B	✓
index-17468f..5d83.js gzip	227 B	227 B	✓
link-db223d9..dbd7.js gzip	1.61 kB	1.61 kB	✓
routerDirect..924c.js gzip	284 B	284 B	✓
withRouter-7..c13d.js gzip	284 B	284 B	✓
Overall change	8.27 kB	8.27 kB	✓

Client Build Manifests

	vercel/next.js canary	btk/next.js patch-1	Change
_buildManifest.js gzip	322 B	322 B	✓
Overall change	322 B	322 B	✓

Serverless bundles

	vercel/next.js canary	btk/next.js patch-1	Change
_error.js	1 MB	1 MB	✓
404.html	2.67 kB	2.67 kB	✓
hooks.html	1.92 kB	1.92 kB	✓
index.js	1 MB	1 MB	✓
link.js	1.06 MB	1.06 MB	✓
routerDirect.js	1.05 MB	1.05 MB	✓
withRouter.js	1.05 MB	1.05 MB	✓
Overall change	5.16 MB	5.16 MB	✓

Commit: e8abba0