Paimon tiering best practise note in docs

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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

• I'm willing to submit a PR!