ci: disable Jekyll processing

[nstepien]

gh pages is currently not serving files/directories starting with an underscore, adding a .nojekyll file fixes that.
For reference: https://github.blog/news-insights/bypassing-jekyll-on-github-pages/

Broken file:
https://github.com/adazzle/react-data-grid/blob/gh-pages/assets/_commonjsHelpers-BosuxZz1.js
https://adazzle.github.io/react-data-grid/assets/_commonjsHelpers-BosuxZz1.js

[nstepien]

We still want to validate formatting with prettier, so I'm removing those paths-ignores

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.71%. Comparing base (e2f54fc) to head (9b5fceb).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3603      +/-   ##
==========================================
+ Coverage   89.50%   91.71%   +2.20%     
==========================================
  Files          48       48              
  Lines        3440     3440              
  Branches      658      681      +23     
==========================================
+ Hits         3079     3155      +76     
+ Misses        361      285      -76     

see 13 files with indirect coverage changes