refactor: Enhance docs, add tests in `FibonacciHeap`

[Hardvan]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] 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 :white_check_mark:

Project coverage is 71.08%. Comparing base (8a18d45) to head (823a160).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5979      +/-   ##
============================================
+ Coverage     70.89%   71.08%   +0.19%     
- Complexity     4870     4879       +9     
============================================
  Files           650      650              
  Lines         17529    17529              
  Branches       3378     3378              
============================================
+ Hits          12427    12461      +34     
+ Misses         4635     4598      -37     
- Partials        467      470       +3     

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