URLs for stable version on readthedocs

[rrjbca]

Description

Some urls in the README currently point to the "latest" build on readthedocs. Instead they should point to the "stable" build.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request