Since compatibility will break with introduction of Godot 3.x, a separate version of documentation for Godot 2.x will be necessary for a some time IMO.
Just think of it when I read early 2017 as the planned date for Godot 3.x. By that time my students will already be in half way through their projects, so a sudden change in tutorials and class reference on internet will be problematic?
WDYT?
There's already a branch for 2.1.x, which is used by the /stable and /2.1 versions of the docs:
The master branch is used for the /latest version, which will be then branched off to a 3.0 branch when 3.0 is released, and continue it way towards 3.1, and so on and so forth.
Will there be stable2.x and stable3.x or stable will be following Godot3.x development after release?
/stable will always be the latest stable, but previous stable releases will stay accessible at /2.1, /3.0, /3.1, etc.
So, fixed already as far as I'm concerned :)
If you're writing documentation for students, give them the /2.1 link to ensure they stay on the version used in the course.
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/stablewill always be the latest stable, but previous stable releases will stay accessible at/2.1,/3.0,/3.1, etc.