Add examples of apps with simplified UI

[gogonzo]

part of insightsengineering/teal.transform#24

Don't know yet where to put it, but we are in the middle of massively simplifying the teal::init API and it would be good to have a place where we highlight those changes.

The simplification I am talking about happened in:

• API simplification: data_extract_spec does not need filter_spec and select_spec definitions to provide defaults teal#51
• API simplification: teal::init could accept a data.frame-like objects passed to its data parameter teal#49
  but there are more issues about it and we could expand the new section with the new changes.

Applies to examples and vignettes