[docs] Improve slots and layout customization examples

[noraleonte]

Problem description

Slots and layout customization seems to be a pain point for users.

People seem to be struggling with custom layouts, view renderers, targeting or replacing different slots.

Opening this based on https://github.com/mui/mui-x/issues/12904

Proposed solutions

1. Add new doc examples demonstrating how to create custom slots for various use cases (i.e. create custom shortcut components) or improve current examples
2. Extend the coverage of the customization playground to DateTimeRangePicker

Search keywords:

[LukasTy]

We discussed a few other possible improvements:

1. Have an example of styling Pickers component fully to a certain look that would show how we suggest doing it.
2. Making the customization playground easier to discover/find. It could already be moved somewhere else or linked back from more places.