GH-46599: [C++][Doc][Parquet] Update supported types documentation

[paleolimbot]

Rationale for this change

We now support more types but the documentation suggested that some weren't supported.

What changes are included in this PR?

Documentation was updated to reflect the status of supported types.

Are these changes tested?

No code changes!

Are there any user-facing changes?

No

• GitHub Issue: [C++][Doc][Parquet] Update doc for JSON, UUID #46599

[github-actions]

⚠️ GitHub issue #46599 has been automatically assigned in GitHub to PR creator.

[pitrou]

Thanks @paleolimbot . Just some formatting suggestions.

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 4 benchmarking runs that have been run so far on merge-commit 1ffc766.

There were 68 benchmark results with an error:

• Commit Run on arm64-t4g-2xlarge-linux at 2025-06-02 19:04:08Z
  • tpch (R) with engine=arrow, format=parquet, language=R, memory_map=False, query_id=TPCH-07, scale_factor=1
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-09, scale_factor=1
• and 66 more (see the report linked below)

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 1 possible false positive for unstable benchmarks that are known to sometimes produce them.