Change docs `_generated` folder name to `api`

As discussed on astropy-dev [here](https://groups.google.com/forum/#!topic/astropy-dev/xrQaI8lxp5o),
I think it's nicer to have `api` than `_generated` for our API docs.

@eteq suggested other folder renames [here](https://groups.google.com/d/msg/astropy-dev/xrQaI8lxp5o/PJzhD6hXLxgJ):
1. `_static` -> `static`
2. `_templates` -> `templates`
3. keep `_generated` but then rename to `api` in the html output

1. and 2. I could easily do if others think it's useful ... 3. I don't know how to do.