Create getting started example

[helderco]

Needed ASAP:

• Python SDK Documentation  #3639
• docs: Add Python SDK docs #3658 (comment)

Idea

Test python code against multiple python versions.

Story

Similar to the Go SDK getting started guide.

1. Test one version
2. Loop over multiple versions
3. Use async concurrency for better performance

@vikram-dagger The cowsay example is pretty simple to start with if you'd rather have that as the getting started and use the test story above as a more elaborate guide.

[helderco]

@vikram-dagger The examples repo depends on #3713, and also an upcoming PR for #3686. I'm sharing sooner to help you with the guide.

If you need to run them in the meantime you can use the first PR and follow the instructions to "Run the engine" in #3642 (comment).

Feel free to close this issue when you have what you need for the "Getting started" guide.

[helderco]

Note on concurrency: The Go SDK had a parallelism example in the end that was later removed. It that case there was only one execution at the end so it was possible to let BuildKit do its thing there. In these examples, each version requires a separate build so we do have increased performance when doing concurrency in Python.