[css-layout-api][explainer] add mandatory intrinsicSizes function

[ggrossetie]

Also, fix the code sample where the constraints variable where redefined.

I’m taking this opportunity to ask if there’s a place where I can ask questions about the Layout API. Specifically, about the concept of breakToken, where despite several attempts, the value is always undefined.
To provide some context, I am trying to see if it is possible to implement certain rules from the CSS Paged Media Module Level 3 with the Layout API.

Thanks!

[ggrossetie]

ggrossetie is unknown, they need to link their GitHub account with a W3C account.

I just connected my W3C account to my GitHub account.