Add versioning to docs output

With this PR, the `make AWS_S3_BUCKET=docs-stage.docker.com docs-release` will push both to the root of the site, and to `/v1.0/` - it needs to run the generation twice, as all the links need to be prefixed.

I have not made a UI for it yet - that comes in a later PR - for now, I want to ensure that we have a `/v1.0/` before we make a `/v1.1/`
This should be `GIT_COMMIT` to match `GIT_BRANCH` nicely. :)