Option To Generate "Clean" notebooks for docs / tutorial for Goolge Colab/PaperSpace etc

- I've met with [Jonathan Whitaker](https://datasciencecastnet.home.blog/) who runs a very popular course on AI Art, with thousands of students. He is using nbdev & fastai for his next iteration of course which is to be released in a couple of months, his course is also sponsored by W&B. He has "run in colab" badges on his notebook, but he wants a version of his notebook that is stripped of all directives, and also the option to add colab specific cells or hide certain things for colab (like the pip install bits, etc).
- W&B docs also have "run in colab" on their docs, but they end up repeating themselves because they have a slightly different version of the code that for Google Colab

I see that this is quite a common pattern for people wanting to make rich tutorials and such with nbdev. We have to think about the design a bit. Rough sketch

front_matter:

```
---
copy_nb_path: ....
---
```

directives:
- `#|copy_exclude`: marks cells that should be excluded from the copied notebook.

Open question: should we have a way to exclude markdown? Perhaps this is possible with conditional rendering?

cc: @jph00 @seeM