Have build machinery recognize and "hide" developer notes

[eteq]

We would like to be able to keep "developer notes" in the notebooks as they exist in source form, but strip them out in the rendered notebook. This requires a few things:

1. Agreeing on exactly what constitutes a "developer note". The easiest rule I can see is for cell that begins with Development note/Developer note (case-insensitive?) to be treated such that the whole cell is a developer note.
2. The build machinery, when run, should remove those cells before (or in the process of) executing. So the generated exec* notebooks should come with them removed. Then the html conversion will use that and come with the developer notes stripped out.
3. To facilitate allowing users to download the version with the developer notes removed, we should add a link that allows the exec* to be downloaded by clicking on a download icon or something e.g. the download buttons from Astropy tutorials like http://learn.astropy.org/rst-tutorials/color-excess.html?highlight=filtertutorials

[camipacifici]

See https://github.com/spacetelescope/notebook-ci-actions/pull/15