DOC: Fix table styling in main docs
When switching to nbsphinx, I modified the site's CSS so
that the converted notebook looks decent. This had some
unfortunate changes on tables elsewhere in the notebook.

This change fixes the headers to be left-aligned in the main site,
and right-aligned for the tables generated by `df.style` in the
nbsphinx-converted notebook.

xref https://github.com/pandas-dev/pandas/pull/15581/

Before:
<img width="801" alt="screen shot 2017-04-26 at 7 11 19 am" src="https://cloud.githubusercontent.com/assets/1312546/25437376/c568c4b2-2a5b-11e7-9bb0-f524d87d3813.png">


After:

![screen shot 2017-04-26 at 8 23 43 am](https://cloud.githubusercontent.com/assets/1312546/25437380/cba5a07a-2a5b-11e7-9669-e9e469e3a7e7.png)

@TomAugspurger 

> When switching to nbsphinx, I modified the site's CSS so
that the converted notebook looks decent. This had some
unfortunate changes on tables elsewhere in the notebook.

what is this modification (I am needing to steal for another project).
``nbsphinx`` is very nice btw.

 > what is this modification 

Just the diff here. Rather than applying the `text-align: right` to all `th`s on the site, it only applies to `th`s with the `col_heading` class. `.style` includes `col_heading` in the generated HTML for headers. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=h1) Report
> Merging [#16145](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/61ca02274b898cc2dd15f305a0b885e2f6348dd8?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16145/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16145      +/-   ##
==========================================
+ Coverage   90.83%   90.86%   +0.02%     
==========================================
  Files         159      159              
  Lines       50787    50787              
==========================================
+ Hits        46134    46146      +12     
+ Misses       4653     4641      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.35% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16145?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/16145?src=pr&el=footer). Last update [61ca022...b37c518](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=h1) Report
> Merging [#16145](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/61ca02274b898cc2dd15f305a0b885e2f6348dd8?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16145/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16145      +/-   ##
==========================================
+ Coverage   90.83%   90.86%   +0.02%     
==========================================
  Files         159      159              
  Lines       50787    50787              
==========================================
+ Hits        46134    46146      +12     
+ Misses       4653     4641      -12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <ø> (+0.02%)` | :arrow_up: |
| #single | `40.32% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.35% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16145?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/16145?src=pr&el=footer). Last update [61ca022...b37c518](https://codecov.io/gh/pandas-dev/pandas/pull/16145?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
