docs(readme): replace template flow with fork → branch → PR

[lassebenni]

Summary

• Students were told to click Use this template, but this cohort repo is not a template — it's the cohort-specific assignment repo they're meant to fork.
• Rewrite How to start to spell out the correct flow: fork HackYourAssignment/c55-data-week1 → clone fork → branch → push → open PR back against HackYourAssignment/c55-data-week1:main.
• Add an explicit ⚠️ "do not click Use this template" callout to head off the most common mistake.
• Update the Submission section to point at the cohort repo, not the student's own fork.

Test plan

• Render README on GitHub and confirm the fork URL/PR target read correctly.
• Sanity-check the auto-grader still triggers on PRs from forks (existing workflow, unchanged).