Update pointer to Theano docs.

[rpgoldman]

The "Theano and PyMC" page of the documentation still pointed to the upstream documentation for Theano, which is no longer accessible (or at least not reliably so).

[codecov]

Codecov Report

Merging #4275 (2669afd) into master (edbafaa) will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4275      +/-   ##
==========================================
+ Coverage   87.44%   87.61%   +0.16%     
==========================================
  Files          88       88              
  Lines       14342    14312      -30     
==========================================
- Hits        12542    12539       -3     
+ Misses       1800     1773      -27     

Impacted Files	Coverage Δ
pymc3/distributions/mixture.py	88.31% <0.00%> (-1.04%)	⬇️
pymc3/distributions/distribution.py	94.57% <0.00%> (+0.25%)	⬆️
pymc3/sampling.py	86.21% <0.00%> (+0.70%)	⬆️
pymc3/parallel_sampling.py	86.79% <0.00%> (+0.94%)	⬆️
pymc3/distributions/multivariate.py	83.33% <0.00%> (+2.16%)	⬆️