Fix format script

[kevinAlbs]

Summary

• Update formatting script for Python 3.12
• Add CONTRIBUTING.md
• Relocate design rationale from README.md to integrating.md

Details

The formatting script is updated to address an observed error on Python 3.12:

$ bash etc/format-all.sh
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'pkg_resources'

The error comes from this check:

run_python -c 'import pkg_resources; pkg_resources.require("pipx>=0.17.0,<2.0")'

pkg_resources is removed in Python 3.12. I expect that check is not strictly needed. This check has been removed.

A MONGOCRYPT_PYTHON environment variable is added to specify an override python version. This is documented in a new CONTRIBUTING.md document.

[vector-of-bool]

LGTM