build paddle_python before build paddle_docs

[QiJune]

Fix #6224

[luotao1]

The total time of travis-ci is 41 min, which is longer than teamcity(33 min). Can we find a better way than just add make paddle_python to build the docs?

In my opinion, building docs should not build entire paddle project. Now, when we modify or add markdown or rst files, the travis ci is very fast, and the teamcity is canceled.

[QiJune]

@luotao1 I make a issue to record this, please refer to #6344