[website] Distinguish between markdown cell code blocks and Jupyter notebook code cells

[duanwilliam]

Currently, notebooks are converted into a long Markdown document, where Jupyter code cells are simply rendered as markdown code blocks. However, this means that if a markdown cell contains a code block, this code block is indistinguishable from a Jupyter code cell. There are several possible approaches to solve this issue:

1. we could simply indent markdown code blocks by a small amount. this would be the simplest solution.

2. we could modify the notebook rendering process to factor in the ideas of Jupyter "cells" - this way, we could additionally do things like adding the cell number on the left (idk what it's called, basically the execution order thing tho), or being able to collapse the output as you would in an actual Jupyter notebook.

cc @StoneT2000 what are your thoughts?