Move the developer notes of the notebooks to a uniform syntax

[eteq]

This plan for development comes from discussion in #84 (e.g. https://github.com/spacetelescope/dat_pyinthesky/pull/84#issuecomment-596774232), #91, and several in-person conversations and a corresponding Jira ticket (not visible in this repo). I'm trying to capture the outcome of that here, but relevant parties are free to add something if I've missed it.

The gist is that we settled on a good approach for including developer notes as individual cells in the notebook that are version controlled along with the notebook. At the outset these will just be visible, but as soon as we get a chance to update the build machinery, we will strip these notes from the built notebook. This has the downside that the downloaded notebooks still have dev notes, but the consensus was that this is really a feature not a bug because we should update the notebooks as dev notes are addressed, and until then it's good for the community to see it as a sort of roadmap of things we know would make their lives easier.

We also settled on a reasonable convention for a developer note that: it should be contained within a single markdown cell (not code cell - code examples in a dev note can be done as literal markdown blocks - i.e. surrounded by```). That cell should begin with the text *Developer Note:*.

So what I see as the remaining to-do’s on this are:

• [x] The README should be updated to reflect the dev note syntax and anyone doing technical reviews should be aware of this and add it to the list of things they check for (that dev notes follow the standard above, I mean).
• [ ] Additionally, existing dev notes should be converted to that format (I think many of them are already close just “dev notes” and maybe without the asterisks?)
• [ ] Maybe some of them need the notes from a PR added into the notebook itself, if it's not already in the notebook itself.
• [ ] We should also record in the README the future intent that eventually the dev notes will be hidden (basically just the wording in JDAT-292's last message from @cpacifici)
• [ ] We should create an issue to track the task of actually hiding the notes in the generated HTML (I think this is pretty easy with nbconvert, but that should be discussed in a more focused issue)

[orifox]

So each of the check boxes above should be a separate tasks? And jira tickets?

[eteq]

So each of the check boxes above should be a separate tasks? And jira tickets?

We could break them out into separate issues in jira to track STScI team work on it, or as separate issues here. But I think they're individually small enough (and best done by one person), so I'm inclined to just say we check them off here as they get done and close this issue once they are.