Create a resusable abstraction layers

[unclecheese]

As a Silverstripe CMS developer, I want a reusable codebase that I can use
across a variety of projects to make integration with search-as-a-service
easy and intuitive so that I can reduce costs and create an excellent
search experience.

Acceptance criteria

• Support multiple search-as-a-service implementations through thin abstraction layers (indexing only)
• [ ] There is clear documentation on how to create a new service implementation (e.g. Algolia)
• Support multiple sources of content through thin abstraction layers (e.g. for dataobject implementation)

[unclecheese]

Removing documentation ACs so they can all be part of the same story.

[chillu]

Moving this back into the backlog, I don't think we can call this card done without some basic illustration that it's indeed reusable. The original Algolia implementation has been removed from this codebase.