[Docs][Python] Move Conda instructions to second option in dev docs setup

### Describe the enhancement requested

Conda instructions are the first ones included here: https://arrow.apache.org/docs/developers/python.html#environment-setup-and-build

Given that this has been mentioned as a tricker setup, let's make things easier for newbies and move this section down and start with the other one.

### Component(s)

Documentation