add a meta example for serializing and loading trained models

[karlnapf]

The example should be should be under "base_api/serialization" or similar

Related to #3555, and #4463

see e.g. the unit tests on how to save/load: https://github.com/shogun-toolbox/shogun/blob/develop/tests/unit/base/trained_model_serialization_test.cc

Use the "new" API (see e.g. #4463), and it might be necessary to add a bit of base-api for that

[sparkingdark]

Is it still open @karlnapf