docs(readme): clarify this is the upstream template, not a student repo

[lassebenni]

Summary

• This repo is the upstream template for the Week 1 assignment, but the README was written as if students would land here and "Use this template" themselves. In practice, students fork their cohort-specific repo under `HackYourAssignment` (e.g. `c55-data-week1`) and open PRs back to it.
• Replace the student-facing How to start section with a redirect to the cohort repos under `HackYourAssignment`.
• Add an For instructors / track maintainers section documenting the cohort-spinup flow (use the GitHub Use this template button with `HackYourAssignment` as owner and `c-data-week1` as the name) and the rule that assignment/dataset/grader edits land here, not on cohort copies.
• Update Submission accordingly.

Companion PR on the c55 cohort repo: https://github.com/HackYourAssignment/c55-data-week1/pulls

Test plan

• Render README on GitHub and confirm the redirect link to the `HackYourAssignment` org works.
• Confirm with cohort instructors that the maintainer flow described matches what they actually do.