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GH-38847: [Documentation][C++] Explicitly note that compute is optional (#43629)



### Rationale for this change

A user didn't know from reading just the compute documentation that compute is an optional feature. We can make that explicit

### What changes are included in this PR?

Added a cross-reference to the optional features section

### Are these changes tested?

No

### Are there any user-facing changes?

No

* GitHub Issue: #38847

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>