DOC: Remove docs code header and use explicit imports in the user guide
I've been discussing with the Binder team, and looks like it'll be possible soon to make the pandas examples in the docs runnable directly from the docs.

I think this will be amazing, but I think it's one more reason to start being explicit and self-contained with the docs. One option is of course that the trickiness we have now on importing things translate to Binder, and we automatically inject code for imports (and may be seeds...).

But my opinion is that the right thing is to start being explicit on what we run in the examples.