Description of change

Move GetBalance.sol to documentation

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Add Solo tests for the added contract

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[x] I have followed the contribution guidelines for this project
[x] I have performed a self-review of my own code
[x] I have selected the develop branch as the target branch
[x] I have commented my code, particularly in hard-to-understand areas
[x] I have made corresponding changes to the documentation
[x] I have added tests that prove my fix is effective or that my feature works
[x] I have checked that new and existing unit tests pass locally with my changes