Adds a basic Git guide to help employees get started with common version control workflows. The update also improves the document formatting so it is easier to read directly on GitHub and uses a more generic remote name in examples.

A new setup script helps employees prepare their development environment with the right identity settings, repository links, useful configuration, and safeguards. This reduces onboarding friction and helps prevent accidental pushes to the main company repository.

- ensures username and email are set
- sets up the main two repositories as remotes (odoo and odoo-dev)
- sets up some other conf (colors)
- optionally adds pull requests on the odoo repo as a third remote
- adds a pre-push hook preventing pushes to odoo by default

Technically, can be used by non-employees. Practically there would be little point.