DOC: Adding offsets to API ref
- [x] closes #24436

Adding Date Offsets API documentation. Added in the same format as other sections. Detected what are properties and what are methods with the `callable` function. The order of the classes is the one already used. The order inside properties and methods is alphabetical.

@jreback In `timeseries.rst` we already have a table with the different offsets: https://pandas-docs.github.io/pandas-docs-travis/timeseries.html#dateoffset-objects Not sure what would you like to add more.

This is how it looks like:
![offsets](https://user-images.githubusercontent.com/10058240/50459176-cfd71300-0960-11e9-9157-eb01f4f62702.png)

CC: @mroeschke @jbrockmendel 