Paimon tiering best practise note in docs

### Search before asking

- [x] I searched in the [issues](https://github.com/alibaba/fluss/issues) and found nothing similar.

### Description

When users use the tiering service to write to paimon, parquet writers use Flink memory however managed memory is not used. Users can set the managed.memory to a small value like 128MB or lower and give that memory back to the JVM heap, to avoid wasting resources.

We can add this as a "best practices section" when starting the tiering service.

### Willingness to contribute

- [x] I'm willing to submit a PR!