Re-introduce trainee interview workshop

[illicitonion]

We have this technical interview workshop

It used to be in Decomposition Sprint 4, but we moved that to the end of tools.

We've also had the following useful feedback on the workshop:

1. It feels quite unguided - interviewing well needs some briefing/training, and we didn't supply that
2. People generally struggled with the exercises more than useful - we should pick some easier ones

We should work out where we want to re-insert this workshop, and how we want to improve those aspects.