[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 <[email protected]>

* Modify styling

* update ^T to ^\top and fix typos

* delete collapse-20

---------

Co-authored-by: Matt McKay <[email protected]>
Co-authored-by: Humphrey Yang <[email protected]>
Co-authored-by: Humphrey Yang <[email protected]>

Author: 3 people