docs: add codecov badge

[vil02]

Continuation of #5098.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 37.13%. Comparing base (295ac4b) to head (782c5b1).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5099   +/-   ##
=========================================
  Coverage     37.12%   37.13%           
- Complexity     2330     2331    +1     
=========================================
  Files           517      517           
  Lines         15645    15645           
  Branches       2984     2984           
=========================================
+ Hits           5808     5809    +1     
  Misses         9548     9548           
+ Partials        289      288    -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.