Switch Markdown engine to markdown-it-py

[uranusjr]

Copying from #16414:

The current Markdown engine does not support fenced code blocks, so it still won’t work after this change. Python-Markdown’s fenced code support is pretty spotty, and if we want to fix that for good IMO we should switch to another Markdown parser. markdown-it-py (the parser backing MyST) is a popular choice for CommonMark support, which is much closer to GitHub-Flavored Markdown which almost everyone thinks is the standard Markdown (which is unfortunately because GFM is not standard, but that’s how the world works).