Add new version of the guide + what's new for 6.0

[lb-]

With the 6.0 release coming really soon, it would be good to prepare the guide for this.

There are a fair few global changes to the UI, headers / breadcrumbs have had tweaks plus some larger changes.

See other issues - https://github.com/wagtail/guide/issues?q=is%3Aissue+%226.0%22