Add section to onboarding documents on DCO and commit signing

[Jochen-Kall]

• setting up git, using --signoff when committing
• DCO checks at github
• setting up tig
• setting up WSL for windows users to have the linux tooling availlable for Windows users

[pahmann]

This is covered in: https://github.com/elisa-tech/wg-automotive#workflow and https://github.com/elisa-tech/wg-automotive#git-newbie-section