Adding Documentation links to the main Opentoonz site

[RodneyBaker]

It would be good to have "Documentation" as one of the options across the top of the Opentoonz website. The link could simply point to the current readthedocs site but I suspect it might be better to figure out a way to get it to point to both Japanese and English documentation and that would likely be best served by a page with the links.

https://opentoonz.readthedocs.io/en/latest/
https://opentoonz.readthedocs.io/ja/latest/

I note that there is an English PDF compilation of the readthedocs Opentoonz documention that can be linked: https://media.readthedocs.org/pdf/opentoonz/latest/opentoonz.pdf

There are links to HMTL and Epub downloads as well via the readthedocs menu.