allow configuring the path of the contents directory

[keewis]

This is the one remaining issue I have before being able to merge the PR that adds try_examples to the xarray docs.

Basically, I would run sphinx once and then retry again after making changes, and if the first run exited with a non-zero exit code the second run would try to execute all the notebooks in the _contents directory. However, if I were to add _contents to exclude_patterns jupyterlite wouldn't be able to find the notebooks in _contents.

As far as I can tell, changing the path from _contents to _build/contents (or _build/jupyterlite-contents) would always succeed, and wouldn't get confused by exclude_patterns. So what I did was:

1. make the path configurable
2. clean up before the next run instead of after the current run

cc @agriyakhetarpal

[keewis]

reading the configuration docs, what contents refers to is quite confusing... on main there's jupyterlite_contents and jupyterlite_ignore_contents, as well as the hard-coded CONTENT_DIR that possibly is only used for try_examples? If so, I might rename the setting to try_examples_content_dir (or jupyterlite_try_examples_content_dir, not sure).