doc: set last version to 0.5

[jakubkoci]

If I understood @genaris correctly, the current version is 0.5. It's a bit confusing because the current docs uses 0.4 as default.

[TimoGlastra]

The reason I kept it as unreleased for now is because I hadn't fully updated the docs yet -- (and kept pushing it forward). And wanted to make sure the guides were correct before setting it as current.

But good nudge to finish it :)