[DOCS][TEST] Documentation and testcases required in list_array.cpp

[sagarpandyansit]

Detailed Description

Link of the file: https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/data_structures/list_array.cpp

Context

• Documentation is required for a better understanding of the code.
• Testing is required to check whether code is correct or not?
• Edge cases must be tested for all methods.

Possible Implementation

• Contributors should write proper documentation and test cases.