Add support for special blocks when panel serves a Markdown file

[MarcSkovMadsen]

As a contributor to the docs of Panel it would be quite helpful if I could quickly render a single markdown document.

To some extent panel serve is the best solution for this. But it cannot handle certain code blocks.

Please support the special codeblocks used by Panel in order of priority

• [ ] Admonitions
• [ ] Tabset
• [ ] Grids

Example rendering

[philippjfr]

The hope was to eventually enable MyST flavored markdown in the markdown pane. When I overhauled the Markdown implementation recently it wasn't quite ready yet, but the MyST world moves fast and we should revisit this.