fixes #17044

1. fixed Python part of the tutorial about using OpenCV XML-YAML-JSON I/O functionality from C++ and Python.
2. added startWriteStruct() and endWriteStruct() methods to FileStorage
3. modifed FileStorage::write() methods to make them work well inside sequences, not only mappings.

it seems that in master branch just the tutorial should be fixed and CV_WRAP should be added to already existing startWriteStruct() and endWriteStruct() declarations in FileStorage class. Then everything works out of the box. But I can check it as soon as the patch gets merged into master and submit another master-specific patch, if needed.

• I agree to contribute to the project under OpenCV (BSD) License.
• To the best of my knowledge, the proposed patch is not based on a code under GPL or other license that is incompatible with OpenCV
• The PR is proposed to proper branch
• There is reference to original bug report and related work
• There is accuracy test, performance test and test data in opencv_extra repository, if applicable patch to opencv_extra has the same branch name.
• The feature is well documented and sample code can be built with the project CMake