Add 'How to add tabs to your inbox' example

[csmeyns]

This change adds example code corresponding to the How to add tabs to your inbox guide.

[csmeyns]

Bit of a shame that it can't have some conversations preloaded so you can test out the tabs, but adding a backend just to call the REST API and do that is probably overkill

Yes agree. Maybe in the future if it turns out that lots of people are using the example, we can opt to add that in after all, but then for now I'll just leave it like this 👌