Added documentation to MiniExcelConverter and fixed warnings in tests and benchmarks projects - #986

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Dereference of a possibly null reference.

Possible null reference argument for parameter 'instance' in 'object? MiniExcelMemberAccessor.GetValue(object instance)'.

Argument of type 'List<MiniExcelColumnMapping>' cannot be used for parameter 'mappings' of type 'List<MiniExcelColumnMapping?>' in 'CellWriteInfo[] AsyncEnumerableWriteAdapter<T>.GetRowValues(T currentValue, List<MiniExcelColumnMapping?> mappings)' due to differences in the nullability of reference types.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Nullability of reference types in value of type 'List<MiniExcelColumnMapping?>' doesn't match target type 'List<MiniExcelColumnMapping>'.

Non-nullable property 'ExcludeNullableType' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.

Non-nullable property 'MemberAccessor' must contain a non-null value when exiting constructor. Consider adding the 'required' modifier or declaring the property as nullable.