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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21440?src=pr&el=h1) Report
> Merging [#21440](https://codecov.io/gh/pandas-dev/pandas/pull/21440?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92e9882d723c57b3135b17bded1a6495fec0ebda?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21440/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21440?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21440   +/-   ##
=======================================
  Coverage   91.89%   91.89%           
=======================================
  Files         153      153           
  Lines       49600    49600           
=======================================
  Hits        45580    45580           
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.29% <ø> (ø)` | :arrow_up: |
| #single | `41.86% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21440?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/21440?src=pr&el=footer). Last update [92e9882...c0b1eef](https://codecov.io/gh/pandas-dev/pandas/pull/21440?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks!