[Doc] provide reference to default time axis label formatter

### What problem does this feature solve?

it is not clear from the doc, how to emulate the default time axis label formatter whereby the formatting logic changes with increasing zoom.

i would like to make a slight modification to that logic, but am not sure where to find the default formatter function to use as a reference

### What does the proposed API look like?

add to the doc

Start by locating the documentation and implementation for the default time-axis label formatter mentioned in the issue. Explain how its formatting changes with increasing zoom and point readers to the formatter reference they need; done when users can find and adapt that logic from the documentation.

Written by the indexing model from the issue text.