[Docs] Enable Discussions

### Describe the enhancement requested

As discussed on the  [mailing list](https://lists.apache.org/thread/wxtcq0xtfcr2rrdlowo93qqvpvhb10pl) we want to enable GitHub discussions for apache/arrow

### Component(s)

Documentation