[Docs] Remove concepts folder

[lfades]

Removed docs/concepts/_data-fetching.md: has no use right now as we have the new data fetching page

Removed docs/concepts/_server-side-and-client-side.md: We don't have an exact match replacement for this but is not something that gets requested in feedback, probably because we do mention the isomorphic nature of Next.js throughout our docs.

[ijjk]

Stats from current PR

Default Server Mode

General

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
buildDuration	10.7s	10.5s	-120ms
nodeModulesSize	47.7 MB	47.7 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
main-HASH.js gzip	6.25 kB	6.25 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
de003c3a9d30..5272.js gzip	10.2 kB	10.2 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
Overall change	56.3 kB	56.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
main-HASH.module.js gzip	4.78 kB	4.78 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
de003c3a9d30..dule.js gzip	6.75 kB	6.75 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
Overall change	51.4 kB	51.4 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
polyfills-HASH.js gzip	26.3 kB	26.3 kB	✓
Overall change	26.3 kB	26.3 kB	✓

Client Pages

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
_app.js gzip	1.24 kB	1.24 kB	✓
_error.js gzip	3.15 kB	3.15 kB	✓
hooks.js gzip	664 B	664 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.03 kB	2.03 kB	✓
routerDirect.js gzip	279 B	279 B	✓
withRouter.js gzip	278 B	278 B	✓
Overall change	7.86 kB	7.86 kB	✓

Client Pages Modern

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
_app.module.js gzip	594 B	594 B	✓
_error.module.js gzip	2.08 kB	2.08 kB	✓
hooks.module.js gzip	370 B	370 B	✓
index.module.js gzip	212 B	212 B	✓
link.module.js gzip	1.48 kB	1.48 kB	✓
routerDirect..dule.js gzip	271 B	271 B	✓
withRouter.m..dule.js gzip	270 B	270 B	✓
Overall change	5.28 kB	5.28 kB	✓

Client Build Manifests

	zeit/next.js canary	lfades/next.js docs/remove-concepts	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/remove-concepts	Change
index.html gzip	917 B	917 B	✓
link.html gzip	926 B	926 B	✓
withRouter.html gzip	915 B	915 B	✓
Overall change	2.76 kB	2.76 kB	✓

---

Serverless Mode (Decrease detected ✓)

General

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
buildDuration	11.3s	11.4s	⚠️ +138ms
nodeModulesSize	47.7 MB	47.7 MB	✓

Client Bundles (main, webpack, commons)

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
main-HASH.js gzip	6.25 kB	6.25 kB	✓
webpack-HASH.js gzip	746 B	746 B	✓
de003c3a9d30..5272.js gzip	10.2 kB	10.2 kB	✓
framework.HASH.js gzip	39.1 kB	39.1 kB	✓
Overall change	56.3 kB	56.3 kB	✓

Client Bundles (main, webpack, commons) Modern

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
main-HASH.module.js gzip	4.78 kB	4.78 kB	✓
webpack-HASH..dule.js gzip	746 B	746 B	✓
de003c3a9d30..dule.js gzip	6.75 kB	6.75 kB	✓
framework.HA..dule.js gzip	39.1 kB	39.1 kB	✓
Overall change	51.4 kB	51.4 kB	✓

Legacy Client Bundles (polyfills)

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
polyfills-HASH.js gzip	26.3 kB	26.3 kB	✓
Overall change	26.3 kB	26.3 kB	✓

Client Pages

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
_app.js gzip	1.24 kB	1.24 kB	✓
_error.js gzip	3.15 kB	3.15 kB	✓
hooks.js gzip	664 B	664 B	✓
index.js gzip	222 B	222 B	✓
link.js gzip	2.03 kB	2.03 kB	✓
routerDirect.js gzip	279 B	279 B	✓
withRouter.js gzip	278 B	278 B	✓
Overall change	7.86 kB	7.86 kB	✓

Client Pages Modern

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
_app.module.js gzip	594 B	594 B	✓
_error.module.js gzip	2.08 kB	2.08 kB	✓
hooks.module.js gzip	370 B	370 B	✓
index.module.js gzip	212 B	212 B	✓
link.module.js gzip	1.48 kB	1.48 kB	✓
routerDirect..dule.js gzip	271 B	271 B	✓
withRouter.m..dule.js gzip	270 B	270 B	✓
Overall change	5.28 kB	5.28 kB	✓

Client Build Manifests

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

Serverless bundles Overall decrease ✓

	zeit/next.js canary	lfades/next.js docs/remove-concepts	Change
_error.js gzip	233 kB	233 kB	⚠️ +19 B
404.html gzip	1.32 kB	1.32 kB	✓
hooks.html gzip	956 B	956 B	✓
index.js gzip	233 kB	233 kB	-188 B
link.js gzip	243 kB	243 kB	-211 B
routerDirect.js gzip	241 kB	241 kB	-208 B
withRouter.js gzip	241 kB	241 kB	⚠️ +16 B
Overall change	1.19 MB	1.19 MB	-572 B