Retire hoverxref extension in favor of RTD link preview

**Current Problem**
We will need to eventually update RTD to enable [link previews](https://docs.readthedocs.com/platform/stable/link-previews.html) because the [sphinx-hoverxref](https://github.com/readthedocs/sphinx-hoverxref/) extension is obsoleted. Our docs meet the requirements of RTD [Detection logic](https://docs.readthedocs.com/platform/stable/reference/main-content-detection.html#detection-logic) but for our ``pydata-sphinx-theme`` popups don't work (when I switch the ``theme to sphinx-rtd-theme`` the popups work).

**Proposed Solution**
I'm hoping that https://github.com/readthedocs/addons/pull/667 for https://github.com/readthedocs/addons/issues/666 solves this for us. The fix is merged but not sure when RTD site starts using it.

**Additional context**
Changes to our docs are in [this reverted commit](https://github.com/dwavesystems/dwave-ocean-sdk/pull/423/changes/8559794940c7be84d250aa610c6e965e272f7977)