DOC: Add missing plots for discrete and continuous distributions (#2871)

* Plots of discrete distributions in the docstrings

* Plots of discrete distributions in the docstrings

* unify style and add missing continuous

* fix several error after building locally

* add line to clarify that formula correspond to pdf/pmf

* fix indent

Author: aloctavodia

RELEASE-NOTES.md

@@ -11,7 +11,7 @@
   Gaussian Process implementation for efficient inference, along with
   lower-level functions such as `cartesian` and `kronecker` products.
 - Added `Coregion` covariance function.
- 
+- Plots of discrete distributions in the docstrings
 
 ### Fixes
 

docs/source/api/distributions/continuous.rst

@@ -29,6 +29,7 @@ Continuous
    VonMises
    Triangular
    Gumbel
+   Logistic
    Interpolated
 
 .. automodule:: pymc3.distributions.continuous