docs: re-structure versioning.

This sorts docs a little differently than what happens currently.
When we introduce the Storage rewrite, it will probably be 0.9.0, which would end us up with this:

```js
'<=0.7.1': ['gcloud', 'datastore', 'storage'],
'>0.7.1 <0.9.0': ['gcloud', 'datastoreWithTransaction', 'pubsub', 'storage'],
'>0.9.0': ['gcloud', 'datastoreWithTransaction', 'pubsub', 'storageWithFiles']
```

We can go on this way, but I thought the re-write below might be easier on us.