adding version control and language selection button

[SDSTony]

I was doing some web-surfing today and eventually found this tool called Materials for MkDocs.

They had some features that I think are nice and could be useful if also provided in Jupyter Book.

1. Version Control

• they provide a button on the header which users can use to surf different versions of a website DOCS, DEMO

2. Language Selection

• they also have a language selection button on the header that can be used to toggle between different language versions of the website DOCS, DEMO

I think that being able to toggle between different versions and languages straight away from the website is an advantage that online books have over hard-copy books.

I'm not quite sure how Material for MkDocs have implemented those two features, but I'm planning on investigating it after I finish up some of my personal stuffs that I'm currently occupied with.

I wonder how the community thinks about these two features...

Thanks~