DOC: Split api.rst in sections
The pandas API reference page is huge. Which makes it difficult for users to find information, and for us to edit.

Given the size of the page, I think it'd make more sense to split the current page `api.rst` into different pages, one per section:
- `api/series.rst`
- `api/functions.rst`
- ...

The document is already divided into sections, I think the current division would make sense with few changes:
- Input/Output
- General functions
- Series
- DataFrame
- Panel
- Index (will include all the Index classes/section in the top level of `api.rst`)
- Scalars
- Date Offsets and Frequencies
- Window
- GroupBy
- Resampling
- Style
- Plotting
- General utility functions
- Extensions

Does anybody think a single page is better? Any other division you think of?