Add the example on downloading reinstantiable flows

[PGijsbers]

See #1165

[PGijsbers]

Though it's quite possible that the GraphQL API will make this example obsolete, but that might take a while. And even then we would still want an example, just using the new interface. GraphQL will not solve the issue on first filtering for reinstantiable flows.

[tiya-9975]

Hi @PGijsbers ! I’d like to work on this documentation example.

My plan is to add a clear example showing:

• how to download a reinstantiable flow using openml.flows.get_flow()
• how to access the underlying sklearn model
• how to reinstantiate the flow locally
• (optional) how to run the reinstantiated model on a small OpenML task

This example can be added under the existing flows documentation or in the examples gallery.

If this approach looks good, I’ll start working on it.
Thanks!