generate-docs.sh should support versioning

[ksdme]

Such as,

• docs/stdlib/collections/
• docs/stdlib/collections/v0.x.y/

[ziflex]

Alternatively, we could just have an additional line comments like “since x.x.x”.
It will keep the documentation easy to maintain.
The only downside of this approach is that if we introduce changes in API documentation will contain only recent version.

[ksdme]

I think we should do both, maintain previous versions while also indicating the minium required version for a function. Doing the versioning thing should be easy as the script would just need to take one additional parameter and change it's output directory to use the one with version suffix, doing the latter would however either require updating comments of each function or using the git tree to find out when the function was first introduced, this however wouldn't indicate stability.