Create a Pull Request Session

The PR content is inside the [synchronising](https://github.com/ARCTraining/cdt_fluids_2022/blob/8cccd0bcdaa5bccd842bce5e0bf255715e26aeee/book/part2/synchronizing-vsc.md), but is an important topic and would be better have one dedicated session for this content.

Break the content in two sessions and improve the PR content.