Update documentation links in README.md

[abhro]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

The repository is no longer under the GitHub jonniedie url, and the documentation is now hosted by SciML.

[jonniediegelman]

Thanks for the update!