docs: use npm ci in CONTRIBUTING.md

[pichlermarc]

Which problem is this PR solving?

we still recommend using npm install in the docs, but we now include a package-lock.json so npm ci provides a better development experience.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.27%. Comparing base (f8ab559) to head (f22a3f0).
Report is 29 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5040      +/-   ##
==========================================
- Coverage   93.39%   93.27%   -0.13%     
==========================================
  Files          46      317     +271     
  Lines         712     8194    +7482     
  Branches      120     1640    +1520     
==========================================
+ Hits          665     7643    +6978     
- Misses         47      551     +504     

see 266 files with indirect coverage changes