Markdown-native toctrees

[kousu]

Hi! I found your research on SO. Thanks so much for trail-blazing this. I was getting ready to figure out how to implement your solution when I noticed that https://sphinx-themes.org/sample-sites/sphinx-book-theme is entirely written in markdown. Here's how they seem to have done it:

• in index.md (instead of index.rst) add
• a code block tagged with type toctree:

 ```{toctree}
 :maxdepth: 1
 :caption: Topic Areas
 
 tutorials/get-started
 customize/index
 content-blocks
 notebooks
 launch
 ``

We can see the full working docs at https://github.com/executablebooks/sphinx-book-theme/tree/master/docs