No, Sphinx is not a wiki system.
The defining feature of a wiki is that users can edit the pages on the fly in the same web app that is presenting the pages. Sphinx generates static content, and does not provide an interface for editing it.
That being said, it may be useful for your use-case.
If you put your Sphinx content in a gitHub repo, you can serve up the rendered html with gitHub pages, and your users/contributors can edit the content with gitHub's interface, OR by cloning the repo and doing the usual PR dance that is used for code.
If your contributors are fairly sophisticated or used to the gitHub workflow, this can work well.
Alternatively, there may well be a wiki system that allows RST as the markup language -- maybe even gitHub's wiki.
-CHB