Project download page does not conform to Apache requirements

Our last two release announcements were rejected on the following grounds:
> Sorry, but the download page does not contain the required links to KEYS, sigs and hashes.
>
> Please see:
>
>
> and the previous section
>
> There is a project download page at:
>
>
> but it is rather out of date and does not have the required links either.
> Note that the ASF releases open source, so the download page must include that.

**Reporter**: [Zoltan Ivanfi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=zi) / @zivanfi

_{**Note**: *This issue was originally created as [PARQUET-1298](https://issues.apache.org/jira/browse/PARQUET-1298). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Inizia dalla pagina di download del progetto all’indirizzo parquet.apache.org/downloads/ e confrontala con i requisiti di Apache per i link di download delle distribuzioni di release citati nell’issue. Aggiorna la pagina per includere i link richiesti a KEYS, firma, hash e release open source, quindi verifica che la pagina sia aggiornata e conforme a tali requisiti.

Scritto dal modello di indicizzazione a partire dal testo della issue.