Pedagogy: move 00_introduction/40_how_to_draw_a_circle → 06_python_advanced/80_circle_case_study

## Goal

`00_introduction/40_how_to_draw_a_circle.ipynb` is a case study using sympy + numpy + matplotlib to express the same circle three ways (closed, implicit, parametric). With the `00_introduction` restructure (motivation only), case studies move to `06_python_advanced`. The circle case study is a natural capstone after numpy + matplotlib basics.

## Scope

Move `00_introduction/40_how_to_draw_a_circle.ipynb` → `06_python_advanced/80_circle_case_study.ipynb`. Mechanical move (rename + path adjustment + Colab badge URL update). No content rewrite in this issue.

Suggested ordering within `06_python_advanced/`:

- 60_numpy_basics
- 70_matplotlib_basics
- 80_circle_case_study ← uses 60 + 70 + sympy
- 90_pandas

## Dependency

- Companion content-completion issue must land FIRST (hint + solution + exercises). Then this move is purely mechanical.

## Acceptance

- [ ] File moved.
- [ ] Colab badge URL updated to new path.
- [ ] Internal links / README references updated if any.
- [ ] Notebook still runs from new location.