adding repoproviders documentation

[choldgraf]

This is a documentation update for repository providers, it does these main things:

1. Updates our table of repoprovider specs to include recent additions
2. Adds a new page dedicated to repository providers, and moves a section of the API docs there (with the goal of making this information more discoverable)
3. Adds a section on adding a new repository provider, so it's easier to
   provide guideance on PRs like Add Dataverse to UI. Fixes #900 #969

What do folks think?

[betatim]

Should we add a step that it is highly likely that your first step would be to add a new content provider to repo2docker?

[choldgraf]

accepted your comments and added a note about content providers. I also realized we don't have a similar section for how to add a new content provider in the repo2docker docs, so I opened jupyterhub/repo2docker#823 too

[nuest]

I re-visited a repo provider I've added (see #951) and found that I had to make changes to binderhub/static/js/index.js for the example text and binderhub/templates/index.html for the dropdown entry. AFAICS that's missing from the steps.

[ The JS file reminds me of the need for a proper API to generate the UI... still very good to document the current state though! ]

[choldgraf]

@nuest aren't both of those addressed here: https://github.com/jupyterhub/binderhub/pull/1017/files#diff-cbeb18fbf4548b84e49c010cf02f439dR57 ? Let me know if I'm missing something!

(and I totally agree about the need to do less hard-coding etc)

[nuest]

@choldgraf My bad, those files are indeed included in that changeset!

[choldgraf]

anything else to do here? If we can get this in then we'll ask #967 and #969 to document themselves and then we can get those merged too

[choldgraf]

I thought about it a bit more and decided to add repository provider information to the table, the new version is here:

https://643-89419368-gh.circle-artifacts.com/0/html/developer/repoproviders.html#supported-repoproviders

[betatim]

Merging this now. We can keep tweaking it in a new PR.