chore: Add Codecov step for CI workflow

[filzrev]

This PR intended to fix #9853.

Previously I've removed Codecov step from CI workflow.
Because it's behavior is unstable.

Codecov v5 support new Tokenless Uploads for Public Repositories feature.
So it's expected to be works as expected without explicitly specify CODECOV_TOKEN token.

Note: It might requires additional settings on Codecov site with admin privileges to use tokenless upload.

• https://app.codecov.io/gh/dotnet/docfx
• https://docs.codecov.com/docs/codecov-tokens#uploading-without-a-token

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.39%. Comparing base (fe673ec) to head (db8e24f).
Report is 492 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10456      +/-   ##
==========================================
+ Coverage   74.31%   79.39%   +5.07%     
==========================================
  Files         536      547      +11     
  Lines       23189    23620     +431     
  Branches     4056     4060       +4     
==========================================
+ Hits        17234    18753    +1519     
+ Misses       4853     3697    -1156     
- Partials     1102     1170      +68     

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