Read the Docs moved hosting to readthedocs.io instead of readthedocs.org. Fix
all links in the project.

For additional details, see:

https://blog.readthedocs.com/securing-subdomains/

> Starting today, Read the Docs will start hosting projects from subdomains on
> the domain readthedocs.io, instead of on readthedocs.org. This change
> addresses some security concerns around site cookies while hosting user
> generated data on the same domain as our dashboard.

Update badge link per the documentation:

https://read-the-docs.readthedocs.io/en/latest/badges.html

> If you leave the version argument off, it will default to your latest
> version. This is probably best to include in your README, since it
> will stay up to date with your Read the Docs project: