docs: add time & space complexity for Stacks (#7132)"

[Srikar-Yekabote]

• 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 78.90%. Comparing base (d9f2ac8) to head (2899f7e).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #7165   +/-   ##
=========================================
  Coverage     78.90%   78.90%           
  Complexity     6853     6853           
=========================================
  Files           769      769           
  Lines         22668    22669    +1     
  Branches       4456     4456           
=========================================
+ Hits          17887    17888    +1     
  Misses         4067     4067           
  Partials        714      714           

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[DenizAltunkapan]

@Srikar-Yekabote please format your code with clang