DOC: use mathjax on sphinx - #15469 Exponentially Weighed Windows pages now shows formulas
closes #15469 

So indeed pngmath is deprecated. We have to choose between `sphinx.ext.imgmath` or `sphinx.ext.mathjax` as described below. I would personally go with mathjax (you don't need latex install for building the docs, but it renders only if you are online, but for html docs I think this is OK)

http://www.sphinx-doc.org/en/stable/ext/math.html lgtm (minor comment) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15618?src=pr&el=h1) Report
> Merging [#15618](https://codecov.io/gh/pandas-dev/pandas/pull/15618?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3d699884e26120618bf0bb8869bc07f1e51a2935?src=pr&el=desc) will **decrease** coverage by `-0.03%`.
> The diff coverage is `n/a`.


```diff
@@            Coverage Diff             @@
##           master   #15618      +/-   ##
==========================================
- Coverage   91.02%   90.99%   -0.03%     
==========================================
  Files         143      143              
  Lines       49301    49301              
==========================================
- Hits        44875    44863      -12     
- Misses       4426     4438      +12
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15618?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/3d699884e26120618bf0bb8869bc07f1e51a2935...eee5f987d29b54e71bd7da2ef83dade7254c9055?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :x: |
| [pandas/tools/merge.py](https://codecov.io/gh/pandas-dev/pandas/compare/3d699884e26120618bf0bb8869bc07f1e51a2935...eee5f987d29b54e71bd7da2ef83dade7254c9055?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL21lcmdlLnB5) | `91.76% <0%> (-0.35%)` | :x: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/3d699884e26120618bf0bb8869bc07f1e51a2935...eee5f987d29b54e71bd7da2ef83dade7254c9055?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.96% <0%> (-0.34%)` | :x: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/3d699884e26120618bf0bb8869bc07f1e51a2935...eee5f987d29b54e71bd7da2ef83dade7254c9055?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.87% <0%> (-0.1%)` | :x: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/compare/3d699884e26120618bf0bb8869bc07f1e51a2935...eee5f987d29b54e71bd7da2ef83dade7254c9055?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.25% <0%> (ø)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15618?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15618?src=pr&el=footer). Last update [3d69988...f4684fb](https://codecov.io/gh/pandas-dev/pandas/compare/3d699884e26120618bf0bb8869bc07f1e51a2935...f4684fb3028dd7492d22b87dc2d99de49aaf18b5?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). @jorisvandenbossche ok on merging this? Yep. Fixed the whitespace before merging.

@cdemulde Thanks! 
(btw, you should set your editor settings to not introduce tabs, but 4 spaces instead of tab) https://travis-ci.org/pandas-dev/pandas/jobs/209368323

I restarted this twice and failed at the same point :< The problem with the parallel building is that it just says "waiting for workers" instead of giving output, so difficult to see what is going on. In any case, this worked locally. Will try with more verbose output. maybe add ``-v`` for now? and certainly take out ``-j 2`` as not helping :<