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GH-32335: [C++][Docs] Add design document for Acero (#35320)

### Rationale for this change

The documentation for Acero was incomplete.  This PR refactors the existing documentation and adds several entirely new sections to form a complete design document for Acero.

### What changes are included in this PR?

Some existing documentation is cleaned up.  Acero documentation is moved into its own folder and broken into several pages.

### Are these changes tested?

The documentation is built as part of the CI but I wouldn't say it is fully tested.

### Are there any user-facing changes?

There are not code changes (other than the removal of two legacy methods) but there are many user-facing documentation changes.
* Closes: #32335

Lead-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>