Add 'Render ... Markdown blocks' option to published apps

[cr0t]

Disclaimer: it's a proof-of-a-concept for an idea discussion.

It's an attempt to introduce a “Render … Markdown blocks” feature to the published apps as an option. It shall consider rendering markdown blocks from the original notebook as an addition to the evaluable cells.

Due to the limited understanding of how the published app rendering pipeline works, this commit should be considered only as a proof-of-a-concept, that most likely will be reworked fully.

Recording

https://github.com/user-attachments/assets/8b6838f8-a5fe-4116-ae30-7b09cce7f427

P.S. Eventually, it should also take image and section titles cells and render them.