Issue with nightly build and deploy

[mmcky]

There seems to be an issue with the latest merge via the nightly build and deploy

Exception occurred:
  File "/usr/share/miniconda3/envs/qe-example/lib/python3.7/site-packages/jupyter_book/toc.py", line 77, in add_toctree
    f"Mixed chapters and individual files in `_toc.yml` entry {parent_name}"
ValueError: Mixed chapters and individual files in `_toc.yml` entry docs/index

@choldgraf Is this an issue from a recent software update? The latest commit seems pretty straight forward

[choldgraf]

yep - see the API Changes section here: https://github.com/executablebooks/jupyter-book/releases

the problem is your headers: I think

[mmcky]

thanks @choldgraf I will do some digging.