Add user advice on Dask "best practices" 

configuration, distributed scheduling etc

Promote dask/distributed
Demo efficient approaches for large-scale processing
```[tasklist]
### Tasks
- [ ] https://github.com/SciTools/iris/pull/5190
- [ ] https://github.com/SciTools/iris/issues/5344
```
