Fix a few sphinx warnings for new doc build

[ColCarroll]

Fixed a few errors and warnings while building https://docs.pymc.io/ . I think we have a grace period of a bit longer if we'd like to fix more things.

Little changes:

• pm.diagnostics no longer exists
• stan was not being recognized by the code highlighter in blocks
• location of a few imports

[review-notebook-app]

Check out this pull request on 

You'll be able to see Jupyter notebook diff and discuss changes. Powered by ReviewNB.

[rpgoldman]

If we have a grace period, we should probably fix the various uses of : instead of : in the parameters sections, and I think Sphinx wants the docstrings to have Notes with an "s", not Note.

LMK if you would like to divvy up these tasks.

[codecov]

Codecov Report

Merging #3702 into master will increase coverage by 27.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #3702       +/-   ##
==========================================
+ Coverage   62.38%   89.9%   +27.51%     
==========================================
  Files         134     134               
  Lines       20269   20269               
==========================================
+ Hits        12645   18223     +5578     
+ Misses       7624    2046     -5578

Impacted Files	Coverage Δ
pymc3/distributions/transforms.py	87.65% <ø> (+20.98%)	⬆️
pymc3/distributions/distribution.py	95.25% <0%> (+0.89%)	⬆️
pymc3/step_methods/slicer.py	95.45% <0%> (+1.51%)	⬆️
pymc3/distributions/discrete.py	74.51% <0%> (+1.93%)	⬆️
pymc3/step_methods/hmc/base_hmc.py	95.32% <0%> (+3.73%)	⬆️
pymc3/step_methods/hmc/nuts.py	99.35% <0%> (+4.48%)	⬆️
pymc3/model_graph.py	88.48% <0%> (+5.03%)	⬆️
pymc3/backends/report.py	92.17% <0%> (+5.21%)	⬆️
pymc3/glm/families.py	95.45% <0%> (+7.57%)	⬆️
pymc3/blocking.py	68.62% <0%> (+8.82%)	⬆️
... and 68 more

[ColCarroll]

I probably won't have time to do more of that this week, but I will make time to do reviews. I think our grace period lasts until the next big feature or api change hits, though!