Math rendering issue

[chaozg]

General description of task

There are further equation rendering issues at https://cuqi-dtu.github.io/CUQIpy/user/_auto_howtos/defining_posterior.html ,
https://cuqi-dtu.github.io/CUQIpy/user/_auto_howtos/gibbs.html , and https://cuqi-dtu.github.io/CUQIpy/api/_autosummary/cuqi.implicitprior/cuqi.implicitprior.MoreauYoshidaPrior.html

Definition of Done (Feature/change)

• fix rendering issue without changing the content

Definition of Done (Mandatory)

• [na] Documentation added (docstrings on all public methods/classes)
• [na] Unit tests added/updated (and passing!)
• Code reviewed and approved by 2 CUQI-maintainers
• Online documentation is rendered correctly (esp. math)

Definition of Done (Optional)

• [na] HowTo/tutorial added