Trait mutual exclusivity

## What

## Why


```[tasklist]
### Tasks
- [ ] https://github.com/OpenAssetIO/OpenAssetIO-TraitGen/issues/64
- [ ] https://github.com/OpenAssetIO/OpenAssetIO-MediaCreation/issues/93
```
Note: It's possible we wont actually do the validation side of this, because these problems _can_ just be solved by doing the documentation issues.