Python: Update dev setup readme (#6797)

### Motivation and Context

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The DEV setup readme does not clearly outline which workspace and
formatter to use in VSCode. And some instructions also show
inconsistencies in the project folder structures.

### Description

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Update the README to make things more obvious and consistent.

### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone :smile: