Allow indexing of segmented content (fluent, subsites)

[unclecheese]

As a Silverstripe CMS developer, I want to be able to store segmented content types in search so that end users of my project can search for content that is isolated to specific domains of the website.

Acceptance criteria

• Works with subsites
• Works with fluent
• Segmentation of content should be implicit (i.e. it requires no extra steps from a developer to filter it)
• There is clear documentation on how to use Fluent and Subsites with search as a service