docs: Convert and add more documentation examples from LIT tests

[asraa]

c/f #1987

Thank you Jeremy!

[j2kun]

An example: operation-balancer doesn't have a doctest, and so we could add a new doctest.mlir and add the doctest directive here: https://github.com/google/heir/blob/7aa0fe8285e0df05f0c3bd94e432e650fad556a0/lib/Transforms/OperationBalancer/OperationBalancer.td#L30

Similar to existing doctest examples like this one: https://github.com/google/heir/blob/7aa0fe8285e0df05f0c3bd94e432e650fad556a0/lib/Transforms/Secretize/Passes.td#L34