Add watchdog to the list of dependencies

[severo]

Currently, after installing doc-builder (I did it with version 0.3.0 from pip, but I understand it has not changed), if I run doc-builder preview, I get:

ImportError: Please install `watchdog` to run `doc-builder preview` command.
You can do so through pip: `pip install watchdog`

watchdog should be added to setup.py

[sgugger]

No, it's not a hard-dependency because it's only needed for doc-builder preview. We could add an extras for preview however with this dependency.