docs: explain how to prevent global styles pollution

[abenhamdine]

Add explanation in docs (customize-theme) to how to prevent global styles pollution by configuring webpack.
Based on #4331 (comment)
This solution looks the best for people using webpack.

Add also another solution based on #4331 (comment)

Should help to decrease noise in #4331 ;)

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[codecov]

Codecov Report

Merging #11929 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #11929   +/-   ##
=======================================
  Coverage      92%      92%           
=======================================
  Files         200      200           
  Lines        5192     5192           
  Branches     1451     1460    +9     
=======================================
  Hits         4777     4777           
- Misses        409      411    +2     
+ Partials        6        4    -2

Impacted Files	Coverage Δ
components/transfer/index.tsx	87.41% <0%> (ø)	⬆️
components/time-picker/index.tsx	81.13% <0%> (ø)	⬆️

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[afc163]

Thx @abenhamdine, this workaround seem to a bit twist though we can suggest with it until find another solution.

[abenhamdine]

this workaround seem to a bit twist though we can suggest with it until find another solution.

Yes, you're right. IMHO the solution should be to use a less variable to optionnaly scope the global styles, I failed to find time to send according PR until now.

[afc163]

ref #9363