docs: first-run path — install to first useful reading

[huang195]

Feature Description

The README leads with the local-tool story well. Below it, the docs are organised around the
platform — plugin catalog, plugin contracts, Kubernetes, SPIFFE — rather than around a person
installing Cortex on a laptop for the first time.

Proposed Solution

• One path that works start to finish: install → observe → read the numbers → stop/remove
• Real output shown, not paraphrased
• Every command verified against v0.9.0
• Links out to the new surfaces rather than duplicating them

Each feature issue documents its own surface; this is the spine that connects them.

Additional Context

Part of the Cortex v0.9.0 release. Out of scope: platform doc corrections #856, #671, #740.