docs: use css-minimizer-webpack-plugin

[sibiraj-s]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

Prefer css-minimizer-webpack-plugin to optimize-css-assets-webpack-plugin

Breaking Changes

• None

Additional Info

• None

[codecov]

Codecov Report

Merging #624 into master will increase coverage by 17.61%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #624       +/-   ##
===========================================
+ Coverage   71.12%   88.74%   +17.61%     
===========================================
  Files          10       10               
  Lines         613      613               
  Branches      160      160               
===========================================
+ Hits          436      544      +108     
+ Misses        161       66       -95     
+ Partials       16        3       -13     

Impacted Files	Coverage Δ
src/loader.js	90.74% <0.00%> (+8.33%)	⬆️
src/CssModule.js	62.68% <0.00%> (+10.44%)	⬆️
src/index.js	91.52% <0.00%> (+13.13%)	⬆️
src/utils.js	88.46% <0.00%> (+34.61%)	⬆️
src/CssDependency.js	100.00% <0.00%> (+65.62%)	⬆️
src/CssLoadingRuntimeModule.js	96.87% <0.00%> (+96.87%)	⬆️

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[alexander-akait]

Big thanks!