change doc to docs

A simple example on how to use this package:

```python

from csst_proto.top_level_interface import flip_image

from csst_proto.api import flip_image

from astropy.io import fits

# read an L0 image

C7主巡天流水线接口

================

Change list

    - ``top_level_interface.py`` 文件改名为``api.py``, 这个文件中用于导入每个python package与流水线的接口函数

    - 接口定义更新, 每个开发模块需要把接口函数命名为``base_``开头的函数，例如``base_phot``

Example

    .. code-block:: python

        def base_distortion(input_file, output_file, )