Which issue does this PR close?

• part of [Epic] Better / Improved Documentation, Tutorials and Examples #7013
• Follow on to Complete moving PhysicalOptimizer into datafusion-physical-optimizer #14300 (comment) with @berkaysynnada
• Related to Minor: Update documentation about crate organization #14304

Rationale for this change

DataFusion is largely intended to be used via the datafusion crate. It is split into multiple subcrates largely for project organization rather than for users to use subcrates

The examples currently use many of the datafusion subcrates (e.g. datafusion_common) directly. This is non ideal because

1. It may confuse users who try to follow the examples and lead them incorrectly towards using the subcrates directly
2. It may potentially mask issues where a struct is not publicly exported

Thus I think our examples should only use the DataFusion crate

What changes are included in this PR?

1. Update the examples to use (only) datafusion
2. Re-export ApplyOrder in datafusion::optimizer to make it easier to use
3. Re-export datafusion_physical_optimizer as datafusion::physical_optimizer for consistency

Are these changes tested?

Yes, by CI

Are there any user-facing changes?

1. Better examples
2. Users can use datafusion::optimizer::ApplyOrder in addition to what they currently do use datafusion::optimizer::optimizer::ApplyOrder