Allow nested dataframes to be expanded by default in HTML samples

When `saveDfHtmlSample` is used on objects with nested dataframes to save HTML samples, the resulting HTML samples are collapsed by default. To expand the nested dataframes, the user needs to click on them. For some samples, it makes documentation less convenient to read.


We need to:
- define and implement a proper way to make HTML samples with nested dataframes expanded by default where we need it;
- use this implementation to expand existing HTML samples in the website documentation (where expanding improves readability).