[IDEA] Add Open in GitHub Codespaces option

Love this project! I'm working my way through the book, and this helps me immensely.

I work on the GitHub product Codespaces which provides another way of running the examples here without doing anything locally, and for free since we give away a number of hours of use every month. I have been using Codespaces to work through the examples, using the VS Code Jupyter extension, and they work great. To make this a super-smooth experience two things would need to happen:

- a .devcontainer/devcontainer.json file added to the repo to specify that the VS Code extension should be installed and activated by default
- This button added to the readme [](https://codespaces.new/ageron/handson-ml3?quickstart=1)

What do you think?