Copy executable docs listed in _toc.yml over to /files

[psychemedia]

The demo executable-book PR executablebooks/sphinx-book-theme#558 adds a "Launch in Jupterlite" button to Jupyter Book.

At the moment, the user must manually copy over notebook files to the distribution.

Ideally,jupyterlite-sphinx should copy over executable documents (that can be legitimately launched in JupyterLite) that are listed in _toc.yml over to the distribution directory (eg _build/html/lite/files). There also needs to be a mechanism to copy over additional files referenced from the notebooks.

I'm a little wary that a blanket "copy over all the contents of a specified directory" is too heavyweight, and might copy over unpublished book files that are not listed in _toc.yml over to the JupyterLite file tree.

[psychemedia]

PR 944ccf4 seems to support a jupyterlite_contents setting in conf.py that will copy path contents over to the distribution.

It might perhaps be handy to be able to ignore specified files in that directory too?

[psychemedia]

Contents can be copied over to files based on glob pattern matching in 6730bef . Would it also make sense to see if the match pattern is actually a file path, eg to a _toc.yml type file, and use that as basis for copying over files?