Multilingual collaboration support

[GindaChen]

(Drafting/Discussion needed...)

During the development of Wuhan2020 in group translation, we realize it is pretty tedious to coordinate (arbitrary size) collaborators to translate documents. Here are some of the problems:

1. Adding a language. When adding a language, we typically edit 5 files in site_config and copy the information from one language to the other. The hierarchical structure and manual process is prone to error.

2. Translate and add a document. Adding a document also needs to manually add an entry to the side menu.

3. (Harder) ** Most translator want to navigate the page in the language they choose but not force jump to default language**. Currently our policy is to force jump to the default language if a page does not exist. For translators, they would like to see a notification that this documentation has not been translated, and ideally there is a button for them to raise an issue and guide them how to translate this page. This workflow would be so much efficient for the contributor.

Relates to wuhan2020/wuhan2020.github.io#151