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GH-34638: [C++][Docs] Add documentation for minimal build flags (#34693)



### Rationale for this change

There's an ongoing effort to support smaller builds of Arrow C++ for simple format integration so explicitly documenting the necessary flags would be useful. Also, some recent changes to the build system may require disclosure.

### What changes are included in this PR?

Adds documentation to `cpp/building.rst` regarding:
- Minimal cmake flags for basic format support
- Recent changes to `ARROW_COMPUTE` and how compute kernels are included in builds.

### Are these changes tested?

N/A

### Are there any user-facing changes?

N/A
* Closes: #34638

Authored-by: benibus <bpharks@gmx.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>