DOC: Confusing warning on unstable version when URL points to stable

[pllim]

See screenshot:

Probably needs tweaking by RTD admin on the admin panel.

It thinks "stable" is https://pydata-sphinx-theme.readthedocs.io/en/v0.15.4/ but 0.16.0 is available on PyPI.

[drammock]

oops, either our release checklist is incomplete/wrong, or someone missed a step. I'll fix.

[pllim]

Thanks for the quick fix!