Restructure documentation navigation

[tombentley]

We're working on an Kubernetes operator and consequently are likely to end up with two (or three) separate guides, rather than the current monolithic guide. These guides will be tied to a specific Kroxylicious release. This will mean we need to restructure the website's navigation to support multiple guides.

One way to do this would be to have a per-version "docs landing page", which then has links to the different sets of documentation applicable to that release (and maybe the Javadoc too -- see #45). The "Docs" item in the header would remain the same (expanding into a list of versions) but each version would link to the corresponding landing page. We'd need to decide how to handle the existing URLs (e.g. the current monolithic docs live at https://kroxylicious.io/docs/v0.10.0/, which is the intuitive URL for the per-release docs landing page).