frontend/help: send /help links to MAJOR.MINOR instead of full version
Created by: bobheadxi
Before: https://docs.sourcegraph.com/@v3.39.1/dev
After: https://docs.sourcegraph.com/@3.39/dev
This allows us to "patch" docs for released versions.
Related: https://github.com/sourcegraph/sourcegraph/pull/35590
Test plan
Unit tests, made sure generated links e.g. the above works