Create org-modified unified contributing guide and related coding standards

### Description

We have many places for this like in:
- https://scancodeio.readthedocs.io/en/latest/contributing.html
- See https://github.com/search?q=org%3Aaboutcode-org+contributing+language%3AreStructuredText&type=code&l=reStructuredText

We need to have a single org-wide document that unifies all these practices with a single set of documentation.

Some other recommendations to add:
- small, focused PRs
- adopting existing code style in a repo
- avoiding the introduction of new dependencies unless absolutely needed

The goal is to start with python-inspector as a base repo to refine this.

Also to consider is how to update tools used across repos for this, like black vs. ruff and use the skeleton for this or scripts.