Simplify how versions are configured in front matter

Simplify how versions are configured in front matter, especially for `next/_index.html`. This is what the front matter looks like now:

```yaml
---
title: v3.5-DRAFT docs
cascade:
version: next
versName: &name v3.5-DRAFT
git_version_tag: v3.5.0-beta.4
page_warning: the documentation is in **DRAFT** status.
linkTitle: *name
---
```

I'd like to be able to use `version` in the **Install** page, for example, but I can't atm because it is currently the permalink alias. I'm leaning more and more towards #252, as a partial way of contributing to a solution to this issue too.

Other ideas:

- Separate out the status as, e.g., `versStatus: DRAFT`. For older doc versions we could use `ARCHIVE`

Related: #252, #296, #300, #302, #304