Add `plot_gp_dist` to documentation

[williambdean]

Cannot find this function in the documentation. Might be the whole module missing.

Ref:https://www.pymc.io/search.html?q=plot_gp_dist

[williambdean]

I was looking for this again today 😆

[ricardoV94]

I was looking for this again today 😆

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