docs: Migrate from npm to yarn

[vdusek]

• Migrate from npm to yarn for consistency.
• Same setup as in Crawlee.
• All 3 Python documentation projects (crawlee-python, apify-sdk-python, apify-client-python) now use an identical yarn setup.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.01%. Comparing base (a78478e) to head (94113cf).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #584   +/-   ##
=======================================
  Coverage   76.01%   76.01%           
=======================================
  Files          42       42           
  Lines        2468     2468           
=======================================
  Hits         1876     1876           
  Misses        592      592           

Flag	Coverage Δ
integration	68.96% <ø> (ø)
unit	64.58% <ø> (ø)

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