[docs] Added related links to next.config.js

[lfades]

You may feel a little lost in the sidebar for next.config.js if you end up in one of its sections but haven't checked the introduction to it.

Also added links to sections related to specific configs.

[ijjk]

Stats from current PR

Default Server Mode

General

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
buildDuration	13.7s	13.2s	-488ms
nodeModulesSize	48.9 MB	48.9 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
main-HASH.js gzip	5.11 kB	5.11 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
4952ddcd88e7..54d3.js gzip	4.68 kB	4.68 kB	✓
commons.HASH.js gzip	4.06 kB	4.06 kB	✓
de003c3a9d30..fffc.js gzip	13.7 kB	13.7 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
Overall change	67.5 kB	67.5 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
main-HASH.module.js gzip	4.17 kB	4.17 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
4952ddcd88e7..dule.js gzip	5.56 kB	5.56 kB	✓
de003c3a9d30..dule.js gzip	12.5 kB	12.5 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
Overall change	62.1 kB	62.1 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
polyfills-HASH.js gzip	4.76 kB	4.76 kB	✓
Overall change	4.76 kB	4.76 kB	✓

Client Pages

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_app.js gzip	1.34 kB	1.34 kB	✓
_error.js gzip	4.07 kB	4.07 kB	✓
hooks.js gzip	779 B	779 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.9 kB	2.9 kB	✓
routerDirect.js gzip	283 B	283 B	✓
withRouter.js gzip	282 B	282 B	✓
Overall change	9.87 kB	9.87 kB	✓

Client Pages Modern

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_app.module.js gzip	769 B	769 B	✓
_error.module.js gzip	3.06 kB	3.06 kB	✓
hooks.module.js gzip	371 B	371 B	✓
index.module.js gzip	212 B	212 B	✓
link.module.js gzip	2.47 kB	2.47 kB	✓
routerDirect..dule.js gzip	273 B	273 B	✓
withRouter.m..dule.js gzip	272 B	272 B	✓
Overall change	7.42 kB	7.42 kB	✓

Client Build Manifests

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_buildManifest.js gzip	61 B	61 B	✓
_buildManife..dule.js gzip	61 B	61 B	✓
Overall change	122 B	122 B	✓

Rendered Page Sizes

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
index.html gzip	1.02 kB	1.02 kB	✓
link.html gzip	1.03 kB	1.03 kB	✓
withRouter.html gzip	1.01 kB	1.01 kB	✓
Overall change	3.06 kB	3.06 kB	✓

---

Serverless Mode

General

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
buildDuration	14.2s	14.2s	-7ms
nodeModulesSize	48.9 MB	48.9 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
main-HASH.js gzip	5.11 kB	5.11 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
4952ddcd88e7..54d3.js gzip	4.68 kB	4.68 kB	✓
commons.HASH.js gzip	4.06 kB	4.06 kB	✓
de003c3a9d30..fffc.js gzip	13.7 kB	13.7 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
Overall change	67.5 kB	67.5 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
main-HASH.module.js gzip	4.17 kB	4.17 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
4952ddcd88e7..dule.js gzip	5.56 kB	5.56 kB	✓
de003c3a9d30..dule.js gzip	12.5 kB	12.5 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
Overall change	62.1 kB	62.1 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
polyfills-HASH.js gzip	4.76 kB	4.76 kB	✓
Overall change	4.76 kB	4.76 kB	✓

Client Pages

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_app.js gzip	1.34 kB	1.34 kB	✓
_error.js gzip	4.07 kB	4.07 kB	✓
hooks.js gzip	779 B	779 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.9 kB	2.9 kB	✓
routerDirect.js gzip	283 B	283 B	✓
withRouter.js gzip	282 B	282 B	✓
Overall change	9.87 kB	9.87 kB	✓

Client Pages Modern

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_app.module.js gzip	769 B	769 B	✓
_error.module.js gzip	3.06 kB	3.06 kB	✓
hooks.module.js gzip	371 B	371 B	✓
index.module.js gzip	212 B	212 B	✓
link.module.js gzip	2.47 kB	2.47 kB	✓
routerDirect..dule.js gzip	273 B	273 B	✓
withRouter.m..dule.js gzip	272 B	272 B	✓
Overall change	7.42 kB	7.42 kB	✓

Client Build Manifests

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_buildManifest.js gzip	61 B	61 B	✓
_buildManife..dule.js gzip	61 B	61 B	✓
Overall change	122 B	122 B	✓

Serverless bundles

	zeit/next.js canary	lfades/next.js docs/next-config-related-links	Change
_error.js gzip	70.9 kB	70.9 kB	✓
hooks.html gzip	1.05 kB	1.05 kB	✓
index.js gzip	71.3 kB	71.3 kB	✓
link.js gzip	80.6 kB	80.6 kB	✓
routerDirect.js gzip	78.2 kB	78.2 kB	✓
withRouter.js gzip	78.3 kB	78.3 kB	✓
Overall change	380 kB	380 kB	✓

Commit: e29abeb

[chibicode]

Looks good to me!