docs: add an example demonstrating `define_units()` usage

[jrycw]

Hello team,

As we know, define_units() powers GT.fmt_units() behind the scenes, but it seems to lack a clear example for users to quickly grasp its usage.

I’d like to propose an example that demonstrates how to use define_units() to render a string with unit annotations as a subtitle in the table header. This example highlights that define_units() can be used independently and within components that don’t yet support unit annotations (such as the table header).

Below is the final table for reference.