[von_neumann_model] lecture improvements (#533)

* Update von_neumann_model.md

- Set the max-height of cells to none
- Removed `object` after `class Nuemann`
- Adopted `sphinx-proof` enviornments

* Update von_neumann_model.md

 - Kept roman numerals in `class nuemann`.
- Deleted `<style>` setting which does not work

* Update lectures/von_neumann_model.md

Co-authored-by: Humphrey Yang <39026988+HumphreyYang@users.noreply.github.com>

* Modify styling

* update ^T to ^\top and fix typos

* delete collapse-20

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Co-authored-by: Matt McKay <mmcky@users.noreply.github.com>
Co-authored-by: Humphrey Yang <39026988+HumphreyYang@users.noreply.github.com>
Co-authored-by: Humphrey Yang <u6474961@anu.edu.au>

Author: 3 people