docs: Add an explicit example for implementing a custom likelihood.

[daniel-dodd]

The guide for the custom likelihood is rather thorough in explaining keys concepts -- but an explicit example showing some actual code would be nice here.

[sazio]

Hey @daniel-dodd is there any progress on this ? I'm currently interested in implementing a custom likelihood

[github-actions]

This issue has been marked as stale because it has been open for 7 days with no activity.

[alexander-reeves]

I am also interested in this - In my case I would like to have a likelihood that depends on a quantity that is computed from the samples of the GP and not from the samples of the GP directly. How can I go about implementing this?