Build docs for stable branch and make default

[oliverholworthy]

Part of https://github.com/NVIDIA-Merlin/Merlin/issues/866

• Builds docs for stable branch in addition to main branch
• Updates default documentation links to point to stable version of docs

[review-notebook-app[bot]]

Check out this pull request on 

See visual diffs & provide feedback on Jupyter Notebooks.

---

Powered by ReviewNB

[github-actions[bot]]

Documentation preview

https://nvidia-merlin.github.io/models/review/pr-1116