Use Libertinus font on RTD

[jwrober]

What should be documented? Is there something wrong in the documentation?
Right now RTD uses a standard sanserif font to display the text. We should follow what we do in the client.

Do you have suggestions?
Now that we have settled on the Libertinus font family, add the sans serif Libertinus font to RTD and use that. Probably the same for the monospace font. This would offer a consistent look on the font front.