DOC: Add favicon to doc pages
This is a suggestion for adding a favicon to the doc pages. Having a favicon makes the pandas browser tabs easier to locate, especially with many open tabs or many pinned tabs.

I used the image from the pandas-dev GitHub organization, cropped some of the bars to make it appear less cluttered in the small favicon format, and resized it to a 32x32 `.ico`-file (the cropped png is attached at the end of this issue).

A few screenshots (pinned and normal tabs, alternating with and without the favicon):

Firefox Quantum
![image](https://user-images.githubusercontent.com/4560057/41294884-19082544-6e27-11e8-90d7-db2992f5d1bd.png)

Chromium
![image](https://user-images.githubusercontent.com/4560057/41294942-40a370e0-6e27-11e8-94a7-29ae473bdc5a.png)

Firefox pre-Quantum
![image](https://user-images.githubusercontent.com/4560057/41295070-9c1c7d22-6e27-11e8-9b4b-04d6e0e9fc57.png)

This is somewhat related to #21376, in that the favicon would change if a new logo is decided upon.

Cropped png
![favicon](https://user-images.githubusercontent.com/4560057/41295398-83b5e16e-6e28-11e8-9daf-353e700e6365.png)


- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
