GH-33976: [Python] Expose pyarrow.acero as public submodule and add to reference docs (#34760)

### Rationale for this change

Finalizing https://github.com/apache/arrow/issues/33976 now the basic functionality for working with Acero has been added and this is used internally: publicly exposing the low-level building blocks.

### What changes are included in this PR? (user facing)

A new `pyarrow.acero` module exposing the Declaration and ExecNodeOptions subclasses (from `pyarrow._acero` cython submodule).

### Are these changes tested?

The functionality was already tested. The existing tests are updated to use the public import.

* Issue: #33976

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>