docs: fix IE11 issues

[layershifter]

This PR fixes two issues:

• Prettier@2 contains incompatible code with IE11 which breaks it, so we can't use CDN build for it, it should be compiled locally by Babel
• in chore: use react-intersection-observer in docs to improve perf #3985 we switched to react-intersection-observer in our docs, however it requires intersection-observer polyfill

[github-actions]

size-limit report

Path	Size
bundle-size/dist/Button.size.js	70.84 KB (0%)
bundle-size/dist/Icon.size.js	33.41 KB (0%)
bundle-size/dist/Image.size.js	66.13 KB (0%)
bundle-size/dist/Portal.size.js	49.9 KB (0%)

[codecov-commenter]

Codecov Report

Merging #4009 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4009   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files         185      185           
  Lines        3245     3245           
=======================================
  Hits         3240     3240           
  Misses          5        5           

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