Add cipher class, cipher tests, enhance docs in 'AtbashCipher.java'

[varnan6]

• [✅] 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

Attention: Patch coverage is 77.77778% with 4 lines in your changes missing coverage. Please review.

Project coverage is 62.95%. Comparing base (2040df8) to head (36fdaf8).

Files with missing lines	Patch %	Lines
...n/java/com/thealgorithms/ciphers/AtbashCipher.java	77.77%	2 Missing and 2 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5690      +/-   ##
============================================
+ Coverage     62.90%   62.95%   +0.04%     
- Complexity     4032     4044      +12     
============================================
  Files           566      567       +1     
  Lines         16012    16030      +18     
  Branches       3081     3084       +3     
============================================
+ Hits          10073    10091      +18     
  Misses         5524     5524              
  Partials        415      415              

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[siriak]

Please update with the latest master branch

[varnan6]

Looks good, thanks!

Thank you!

[varnan6]

Please update with the latest master branch

Done, do check