Add feature to track docs from different branches

[SilleBille]

🚀 Tracking docs from different branches

The current implementation allows reading docs from a Single Branch with multiple dirs that point to different version docs.

This feature would allow projects that base their product version on different branches*.

Have you read the Contributing Guidelines on issues?

Yes

Motivation

After attending ATO2018 conference, @yangshun and I had a discussion regarding this feature.

Pitch

Project ATO has branches ATO1 and ATO2. Each branch has a dir named doc that has that particular version's documentation (ie) ATO1/doc has v1 doc and ATO2/doc has v2 doc. Instead of forcing the project to move all their docs to a single branch, docusaurus should be able to accomodate the above mentioned project structure as well.

[yangshun]

Nice meeting you earlier @SilleBille! I don't have a good suggestion for that kind of branching model because if two docs are on different branches, it feels more correct to generate a website separately for each. Take Bootstrap for example, V4 dos are hosted at https://getbootstrap.com/docs/4.1/ and v3 docs at https://getbootstrap.com/docs/3.3/. They are separate websites. I don't think there are any site generators out there that support generating a site across multiple branches.

cc @endiliey and @JoelMarcey for opinions.

[endiliey]

I am not in a good favor of this issue >.<

Branching is actually a non easy concept and I think it's better to make things simple.

[SilleBille]

@endiliey is right. This feature does makes things a little complicated. To answer @yangshun there is readthedocs which actually supports such a feature.