Documentation: Update TTE vignette to discuss positive censoring using `derive_extreme_event()`

[manciniedoardo]

Please select a category the issue is focused on?

User Guides

Let us know where something needs a refresh or put your idea here!

See discussion here - issue to be fleshed out during 1.5 development cycle.

[github-actions]

This issue is stale because it has been open for 90 days with no activity.

[bms63]

This is being actively discussed in #2952 and in #2988