Separating public and private APIs into different sections/pages in the documentation.

[wangvsa]

1. Check the API page and make sure it documents only the public APIs.
2. Put all private/internal APIs in the developer documentation.
3. Both the publication documentation and the developer documentation have a "developers note" section. Merge them.
   https://pdc.readthedocs.io/en/latest/api.html#developers-notes
4. Suggestion: add a "naming convention" section in the developer documentation.