GH-46537: [Docs][C++] Add RunEndEncodedArray, FlatArray, and PrimitiveArray API Docs

[hiroyuki-sato]

Rationale for this change

The Array section of the API Doc
does not explain FlatArray, PrimitiveArray, and RunEndEncodedArray .

What changes are included in this PR?

Add FlatArray, PrimitiveArray, and RunEndEncodedArray API docs

Are these changes tested?

No. (Document only)

Are there any user-facing changes?

No.

• GitHub Issue: [Docs][C++] No RunEndEncodedArray, FlatArray, and PrimitiveArray API Doc #46537

[github-actions]

⚠️ GitHub issue #46537 has been automatically assigned in GitHub to PR creator.

[hiroyuki-sato]

Attached image is the preview image of this PR.

• Where is a good place to put FlatArray and PrimitiveArray? (This PR puts two class definitions in the base category. This is because two classes are defined in the arrow_base.h file.)
• Run-End Encoded Array too. Should we create create new category?

[raulcd]

@github-actions crossbow submit preview-docs

[raulcd]

@github-actions crossbow submit preview-docs

[github-actions]

Revision: 6041e20

Submitted crossbow builds: ursacomputing/crossbow @ actions-682ed9d41c

Task	Status
preview-docs

[raulcd]

LGTM validated here on the preview-docs:
http://crossbow.voltrondata.com/pr_docs/46540/cpp/api/array.html#_CPPv4N5arrow5ArrayE

[pitrou]

Thanks a lot @hiroyuki-sato ! Please feel free to contribute other documentation improvements.

[hiroyuki-sato]

@raulcd @pitrou Thanks!

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 3cc284f.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 5 possible false positives for unstable benchmarks that are known to sometimes produce them.