Added note on env vars

[molebox]

This PR adds a note on how to access env vars in middleware

Bug

• Related issues linked using fixes: process.env is an empty object in _middleware.ts #30612
• Integration tests added
• Errors have helpful link attached, see contributing.md

Feature

• Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
• Related issues linked using fixes #number
• Integration tests added
• Documentation added
• Telemetry added. In case of a feature if it's used or not.
• Errors have helpful link attached, see contributing.md

Documentation / Examples

• Make sure the linting passes by running yarn lint

[styfle]

Lets simplify this

[styfle]

Actually requesting tim's changes too

[ijjk]

Stats from current PR

Default Build (Decrease detected ✓)

General Overall decrease ✓

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
buildDuration	18.1s	18.3s	⚠️ +166ms
buildDurationCached	3.6s	3.6s	-19ms
nodeModulesSize	339 MB	339 MB	-15 B

Page Load Tests Overall decrease ⚠️

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
/ failed reqs	0	0	✓
/ total time (seconds)	3.012	2.967	-0.04
/ avg req/sec	830.11	842.54	+12.43
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.356	1.397	⚠️ +0.04
/error-in-render avg req/sec	1843.28	1789.79	⚠️ -53.49

Client Bundles (main, webpack, commons)

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.2 kB	42.2 kB	✓
main-HASH.js gzip	28.4 kB	28.4 kB	✓
webpack-HASH.js gzip	1.45 kB	1.45 kB	✓
Overall change	72.2 kB	72.2 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
_app-HASH.js gzip	1.37 kB	1.37 kB	✓
_error-HASH.js gzip	194 B	194 B	✓
amp-HASH.js gzip	312 B	312 B	✓
css-HASH.js gzip	327 B	327 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	350 B	350 B	✓
hooks-HASH.js gzip	635 B	635 B	✓
image-HASH.js gzip	4.45 kB	4.45 kB	✓
index-HASH.js gzip	263 B	263 B	✓
link-HASH.js gzip	1.87 kB	1.87 kB	✓
routerDirect..HASH.js gzip	321 B	321 B	✓
script-HASH.js gzip	383 B	383 B	✓
withRouter-HASH.js gzip	318 B	318 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	13.3 kB	13.3 kB	✓

Client Build Manifests

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
_buildManifest.js gzip	460 B	460 B	✓
Overall change	460 B	460 B	✓

Rendered Page Sizes

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
index.html gzip	532 B	532 B	✓
link.html gzip	545 B	545 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.6 kB	1.6 kB	✓

---

Default Build with SWC (Increase detected ⚠️)

General Overall decrease ✓

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
buildDuration	19.7s	19.9s	⚠️ +188ms
buildDurationCached	3.5s	3.5s	⚠️ +28ms
nodeModulesSize	339 MB	339 MB	-15 B

Page Load Tests Overall increase ✓

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
/ failed reqs	0	0	✓
/ total time (seconds)	2.986	3.007	⚠️ +0.02
/ avg req/sec	837.21	831.43	⚠️ -5.78
/error-in-render failed reqs	0	0	✓
/error-in-render total time (seconds)	1.382	1.368	-0.01
/error-in-render avg req/sec	1809.16	1827.96	+18.8

Client Bundles (main, webpack, commons)

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
450.HASH.js gzip	179 B	179 B	✓
framework-HASH.js gzip	42.3 kB	42.3 kB	✓
main-HASH.js gzip	28.6 kB	28.6 kB	✓
webpack-HASH.js gzip	1.44 kB	1.44 kB	✓
Overall change	72.5 kB	72.5 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
_app-HASH.js gzip	1.35 kB	1.35 kB	✓
_error-HASH.js gzip	180 B	180 B	✓
amp-HASH.js gzip	305 B	305 B	✓
css-HASH.js gzip	321 B	321 B	✓
dynamic-HASH.js gzip	2.38 kB	2.38 kB	✓
head-HASH.js gzip	342 B	342 B	✓
hooks-HASH.js gzip	622 B	622 B	✓
image-HASH.js gzip	4.47 kB	4.47 kB	✓
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	1.91 kB	1.91 kB	✓
routerDirect..HASH.js gzip	314 B	314 B	✓
script-HASH.js gzip	375 B	375 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
85e02e95b279..7e3.css gzip	107 B	107 B	✓
Overall change	13.2 kB	13.2 kB	✓

Client Build Manifests

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
_buildManifest.js gzip	459 B	459 B	✓
Overall change	459 B	459 B	✓

Rendered Page Sizes

	vercel/next.js canary	molebox/next.js add-env-var-info-to-middleware-docs	Change
index.html gzip	533 B	533 B	✓
link.html gzip	546 B	546 B	✓
withRouter.html gzip	527 B	527 B	✓
Overall change	1.61 kB	1.61 kB	✓

Commit: 98d34ab