preparation for document translations

We are translating documents and we need this hack to host the translated documents on Read the Docs.

We are doing translation in this way however RTD doesn't generate `.mo` files from `.po` files. So we have to generate it ourselves. (Or we can add `.mo` binary files to repo but it's nonsense.)
http://sphinx-doc.org/latest/intl.html

After merging, you need to configure settings of RTD.
- Enable `Use virtualenv`
- Enable `Use system packages`
- Specify `Requirements file` `doc/pip_requirements.txt`

That's it.


Also, I sent some pull-request to RTD and it's merged. It enables hosting documents in multiple language on same domain, means we can access both of below:
``http://trafficserver.readthedocs.org/en/latest/``
``http://trafficserver.readthedocs.org/ja/latest/``

To use this feature, you can set this project as one of translations on RTD.
https://readthedocs.org/projects/trafficserver-ja/