Rework episode 9 (conflicts)

Background

• Current episode 9 contains too much other things (GitLab UI etc.) which makes it hard to follow up
• A basic concept such as branches is not explained yet

Proposal

Explain conflicts on the basis of a local Git repository and introduce the concept "branches".

• Explain local branches
• Provide a conflict example on the basis of two local branches
• Switch positions of episodes 8 and 9