docs: separate laptop docs from Kubernetes and platform docs

[huang195]

Feature Description

A laptop user following the docs currently lands on SPIRE and Keycloak setup, which reads as
though it were required.

Proposed Solution

• Two entry points, clearly labelled by audience
• The laptop path never requires reading a Kubernetes page
• Platform docs stay reachable and unbroken
• Existing links redirected or updated

Additional Context

Part of the Cortex v0.9.0 release, stretch scope.