missing pyproject.toml in introduction

Introduction github screenshots and ls listings do not show the pyproject.toml file, which is needed to be able to do `pip install -e .`. There is no mention of it either on the installation step.

Start in the introduction material and review the GitHub screenshots, ls listings, and installation step mentioned in the issue. Update the introduction so pyproject.toml is visible and its role in enabling `pip install -e .` is explained; done means a learner can find and understand it from the installation instructions.

Written by the indexing model from the issue text.