DOC: Cleanup for nbsphinx output
Followup to https://github.com/pandas-dev/pandas/pull/15581

Using the `nbsphinx: hidden` metadata to hide the ouptut, so
readers don't see matplotlib's fc-list warning.

Make the tables monospaced in CSS.
@jreback is this what you meant by aligned?

<img width="537" alt="screen shot 2017-04-08 at 1 36 21 pm" src="https://cloud.githubusercontent.com/assets/1312546/24831578/02f0aa8c-1c62-11e7-8ba2-028b28809621.png">

My change made them use monospace font, but doesn't apply any additional styling. hmm maybe need to right align the numbers
i thought we do that by default  We probably do in the notebook, but this is picking up the CSS from the website. I'll right-align in the CSS. Fixed
 
<img width="666" alt="screen shot 2017-04-08 at 2 18 26 pm" src="https://cloud.githubusercontent.com/assets/1312546/24831770/52838c46-1c66-11e7-8c18-f4f39c3c7f86.png">
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=h1) Report
> Merging [#15951](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/88f585155b6cae58b10f875c481871d510b8c591?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/15951/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #15951   +/-   ##
=======================================
  Coverage   90.99%   90.99%           
=======================================
  Files         145      145           
  Lines       49570    49570           
=======================================
  Hits        45106    45106           
  Misses       4464     4464
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <ø> (ø)` | :arrow_up: |
| #single | `40.56% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15951?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/15951?src=pr&el=footer). Last update [88f5851...99318b7](https://codecov.io/gh/pandas-dev/pandas/pull/15951?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/15951?src=pr&el=h1) Report
> Merging [#15951](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/88f585155b6cae58b10f875c481871d510b8c591?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15951/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15951      +/-   ##
==========================================
+ Coverage   90.99%   90.99%   +<.01%     
==========================================
  Files         145      145              
  Lines       49570    49570              
==========================================
+ Hits        45106    45107       +1     
+ Misses       4464     4463       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <ø> (ø)` | :arrow_up: |
| #single | `40.56% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.03% <0%> (+0.34%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15951?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/15951?src=pr&el=footer). Last update [88f5851...99318b7](https://codecov.io/gh/pandas-dev/pandas/pull/15951?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). @TomAugspurger great! yeah to be honest we should prob default right-align / and fixed decimals for the first thing in the notebook (or I think its the default)?

merge away thanks @TomAugspurger  font looks nice, though warnings are still there :<
 Ah shoot. Looks like you actually have to import `matplotlib.pyplot` to trigger it (I don't see it locally). I'm doing another PR touching the notebook, I'll throw the fix in there.