Update documentation for v6

[GreenImp]

Once v6 is complete we need to update all of the documentation to match.
We will need a migration section, as there are going to be breaking changes between v5 and v6.

• Collate list of changes, and group by severity (e.g. Breaking, minor, etc.)
• Add new features (e.g. output plugin)
• Create Migration section, explaining how to migrate from v5 to v6
• Update API docs
• Retain old v5 docs, and ensure they're still available (probably at /documentation/v5)