[Docs][Python] New contributors guide: add build instructions

### Describe the enhancement requested

In the new contributors guide instructions for Python (https://arrow.apache.org/docs/developers/guide/step_by_step/building.html) we point people to the main instructions (https://arrow.apache.org/docs/developers/python.html#environment-setup-and-build). However, these are more detailed than necessary and it's confusing working out where we got to going from one document to another; we should instead include a minimal version here so new contributors have a simple path to follow.

### Component(s)

Documentation