Documented webpack customization example for typescript

[tko]

Provides a copypastable starting point for a quick start and typescript
for those who want it.

Not sure if fixes #875 as the issue mentions storyshots which I have no experience with.

How to test

1. build new docs
2. browse to http://localhost:8000/configurations/custom-webpack-config/#full-control-mode-default

[codecov]

Codecov Report

Merging #1386 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1386   +/-   ##
=======================================
  Coverage   14.35%   14.35%           
=======================================
  Files         201      201           
  Lines        4613     4613           
  Branches      498      498           
=======================================
  Hits          662      662           
- Misses       3517     3522    +5     
+ Partials      434      429    -5

Impacted Files	Coverage Δ
...codemod/src/transforms/update-organisation-name.js	40.62% <0%> (ø)	⬆️
addons/info/src/components/markdown/htags.js	0% <0%> (ø)	⬆️
.../ui/src/modules/ui/components/layout/dimensions.js	15.62% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
addons/knobs/src/components/types/Object.js	5.81% <0%> (ø)	⬆️
app/react/src/client/preview/reducer.js	0% <0%> (ø)	⬆️
addons/knobs/src/components/PropField.js	10.86% <0%> (ø)	⬆️
addons/info/src/components/markdown/code.js	0% <0%> (ø)	⬆️
...react-native/src/manager/components/PreviewHelp.js	0% <0%> (ø)	⬆️
lib/ui/src/modules/shortcuts/actions/shortcuts.js	6.25% <0%> (ø)	⬆️
... and 8 more

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[shilman]

Nice! Great addition 😄