DOC: update io.rst
![image](https://cloud.githubusercontent.com/assets/953992/24471918/c222f848-1491-11e7-8d17-d76da3204399.png)

redo of the top of the io.rst page. was getting a bit long / disorganized. I added links for the various formats as well. *could* add another column about use (e.g. general, portable, flexible, etc), but maybe getting much

merged. can always add / update later. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15840?src=pr&el=h1) Report
> Merging [#15840](https://codecov.io/gh/pandas-dev/pandas/pull/15840?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/abf16978cfcb1188ca856819204e6bc7500af179?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15840/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/15840?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15840      +/-   ##
==========================================
- Coverage   90.97%   90.96%   -0.02%     
==========================================
  Files         143      143              
  Lines       49429    49429              
==========================================
- Hits        44970    44961       -9     
- Misses       4459     4468       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.71% <ø> (ø)` | :arrow_up: |
| #single | `40.7% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15840?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/15840?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/15840?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.56% <0%> (-0.1%)` | :arrow_down: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
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