[C++] Add Parquet docs

[asfimport]

There is currently zero Sphinx doc for Parquet. I'm adding a stub in ARROW-6630 but we should do more, especially as Arrow benefits from tight integration with Parquet.

Reporter: Antoine Pitrou / @pitrou

Related issues:

• [C++][Doc] Document supported Parquet features (is related to)

_{Note: This issue was originally created as ARROW-6699. Please see the migration documentation for further details.}

[asfimport]

Antoine Pitrou / @pitrou:
cc @wesm

[asfimport]

Wes McKinney / @wesm:
Yes I agree having documentation for parquet-cpp would be a good thing

[asfimport]

Antoine Pitrou / @pitrou:
cc @jonkeane

[github-actions]

This issue has been marked as stale because it has had no activity in the past 365 days. Please remove the stale label or comment below, or this issue will be closed in 14 days. If this improvement is still desired but has no current owner, please add the 'Status: needs champion' label.