Re-introduce trainee interview workshop

We have [this technical interview workshop](https://github.com/CodeYourFuture/CYF-Workshops/tree/main/technical-interviews)

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.