Tuning Guide for Larger-than-memory Queries

### Is your feature request related to a problem or challenge?

It would be great to include an example (under `datafusion-examples`) to illustrate:

1. Show how to configure the DataFusion runtime so that a query can run within a specified memory limit.

2. Provide example queries and walk through interpreting the `EXPLAIN` output, focusing on understanding memory usage and identifying whether and when spilling happens.

3. Describe the relevant configuration options for larger-than-memory queries.

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

Start by reviewing the existing examples under datafusion-examples to understand how runtime configuration and query demonstrations are organized. Add an example that configures a memory limit, demonstrates larger-than-memory queries, and explains EXPLAIN output, memory usage, spilling, and relevant configuration options. Done means the example and tuning guidance cover all three requested areas.

Written by the indexing model from the issue text.