[DOC] Example for using list of columns for valuation when declaring triangle.

[henrydingliu]

Description

The Triangle Constructor can take a list of columns, such as year, month. This is used here. but no public facing example, either in the docstring or in the user guide exists

Is your feature request aligned with the scope of the package?

• Yes, absolutely!
• No, but it's still worth discussing.
• N/A (this request is not a codebase enhancement).

Describe the solution you'd like, or your current workaround.

No response

Do you have any additional supporting notes?

No response

Would you be willing to contribute this ticket?

• Yes, absolutely!
• Yes, but I would like some help.
• No.

[henrydingliu]

assigning to @kennethshsu since he was also surprised by this

[henrydingliu]

Here is a screenshot of the current Triangle user guide, which very briefly describes how to do this, without a concrete example.