docs: Refresh the introduction page

This commit updates the introduction page to focus on Rego and
integrating with OPA (whereas the old introduction content focused on
high-level concepts like policy-decoupling, the data document, etc.)

The new introduction content provides a concise overview of OPA and
then launches into an tour of Rego (by example) that shows the core
concepts in the language. The content ends with an overview of
different ways you can interact with OPA.

The old introduction content has been moved into the Philosophy page
and the How Does OPA Work? page has been removed/replaced by the new
introduction page.

With these changes we can update the frontpage with links to core OPA
introduction and then different use cases (e.g., Kubernetes, Envoy,
etc.)

Signed-off-by: Torin Sandall <torinsandall@gmail.com>