How to create Custom Property Types?

I’m really interested in Custom Property Types feature:

I would like to create custom property types that can be used by other people making mods for a game.
Is there more documentation on this? or an example project?

Also, I’m new to tiled and not familiar with the interface…
How do I open the Property Types Editor (aka the Custom Types Editor)?
as shown here: Custom Properties — Tiled 1.9.2 documentation

To answer my 2nd question: The way to open the Custom Types Editor is under View