Replace sphinx with mkdocs as default docs option, add none option

[chrisjkuch]

Closes #327

This PR allows a ccds user to select mkdocs or none as their code documentation tool. mkdocs is the new default.

If mkdocs is chosen, provides the output of mkdocs new {{ project_name }} in /docs/

If none is chosen, provides no default documentation tool, but keeps an empty /docs/ folder.

Notes

To prevent an explosion of possible configuration options in the product that generates test configs, this PR also switches to simply cycling through all possible values for multi-valued configuration options that should be nominally independent (storage backend, license, including the code scaffold, and docs tool).

[jayqi]

@chrisjkuch do you have a sense of where this PR is at? Having a refreshed design around docs seems like it may be worth including in V2.

I was chatting with @pjbull and the desired goal may not be exactly what this was originally doing though. Given that markdown is much more likely to be useful that RST, it probably makes more sense to replace Sphinx with mkdocs, rather than introducing complexity to have Sphinx as an option. Furthermore, it seems also likely that many CCDS-derived projects don't actually need a docs website, and so it may make sense for the choices to be None or mkdocs. So vs. what you have in this PR, I think that would be dropping the Sphinx option.

[chrisjkuch]

This PR is definitely out of date. I agree that straight-up replacement of Sphinx with mkdocs is best (and I think is captured by #327), while also providing a "none" option (which isn't currently captured in that issue).

[chrisjkuch]

Upon further reflection, it would be more effort to remove sphinx than to leave it in. If we have a strong desire to remove it, I can do so, but the easiest thing is to leave it as a non-default option.

Sphinx has been removed

[chrisjkuch]

@pjbull @jayqi Sphinx is removed, ready for review

[pjbull]

A couple little things, thanks @chrisjkuch