[Future] Switch from hoverxref to default RTD hover-over

RTD has recently developed an addon that replaces hoverxref, so that hover-over tooltips can be enabled for projects via the RTD admin panel.

However, there is currently no configuration possible of this feature (although it's on their roadmap), so it is enabled on every kind of link (including normal hyperlinks).

In addition, we currently can't configure the default styling of the tooltips provided by the RTD addon, and the default colours and fonts chosen are problematic from an accessibility PoV (very low contrast, very small text).

However, once these issues are resolved and we have the ability to customise the feature to be better aligned with our own styling, we should switch from hoverxref to the RTD addon.