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 

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