Created by: weixing02

fix #9515 (closed) fix #9516 (closed) fix #9517 (closed)

1. Move contents from paddle/docs/v2/api/fluid to paddle/docs/fluid/api .

2. Build sphinx tree for paddle/docs/fluid/api.

3. Add commands for compiling fluid (docs/ apis) to paddle/scripts/travis/build_doc.sh and paddle/scripts/docker/build.sh.

4. Add some dependencies to simplify commands for compiling docs and apis:

• add gen_proto_py dependencies for paddle_v2_docs, paddle_v2_docs_cn, paddle_fluid_docs, paddle_fluid_docs_cn
• add gen_proto_py framework_py_proto copy_paddle_pybind dependencies for paddle_v2_apis, paddle_fluid_apis.

5. Adjust commands structure：

• now we could use paddle_docs to generate all the documentations.
• now we could use paddle_apis to generate all the apis.

looks like this:

• paddle_docs:
  • paddle_v2_docs
  • paddle_v2_docs_cn
  • paddle_fluid_docs
  • paddle_fluid_docs_cn
• paddle_apis:
  • paddle_v2_apis
  • paddle_fluid_apis