Markdown Notebooks - Markdown notebook structure should be reproducible

[lramos15]

Testing #101325

1. Create two cells first cell containing ***italics** second cell containing **bold**.
2. The outputted markdown just gets concatenated. I would expect with two different cells it would have a line break in between
3. Reopen this markdown in a markdown notebook, the markdown notebook has issues parsing it. I would expect that since I edited this markdown file in a markdown notebook when I reopen it in the notebook it should be the same structure as how I left it.

[roblourens]

Fixing, need to wait a day for #101461