refactor: Enhance docs, remove `main`. add more tests in `HexaDecimal…

[Hardvan]

…ToDecimal`

• [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 71.72%. Comparing base (c657138) to head (ab4cf9a).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5923      +/-   ##
============================================
+ Coverage     71.68%   71.72%   +0.03%     
- Complexity     4944     4946       +2     
============================================
  Files           650      650              
  Lines         17565    17560       -5     
  Branches       3384     3385       +1     
============================================
+ Hits          12592    12595       +3     
+ Misses         4498     4491       -7     
+ Partials        475      474       -1     

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