docs: add Why Cascade positioning and comparison page

[joshua-temple]

Problem

New users have no single page that answers "is cascade right for me, and how does it relate to the release and promotion tools I already know." That question is currently scattered across the README and several reference pages, which slows adoption decisions.

Change

Adds a "Why Cascade" positioning and comparison page (docs/src/content/docs/comparison.md) and places it first in the sidebar as a decision aid.

The page covers:

• What cascade is, in product terms (a compiler that emits native GitHub Actions you own, not a runtime control plane).
• Who it is for, when to use it, and when not to (mirrors the README non-goals: trunk-based only, you own the deploy logic, never rebuilds per stage, metadata courier).
• A fair, three-axis comparison: release and changelog tools (release-please, semantic-release, Changesets, GoReleaser); promotion and rollout control planes (Argo CD and Rollouts, Kargo, Spinnaker, Octopus, Harness); and CI-as-code generators (projen, Dagger, Earthly). Differences are framed as different goals, and complementary use is called out where it applies.
• An accurate list of what cascade generates today.

Verification

• Internal links use the /cascade/... base convention and point only to existing slugs.
• Page claims are scoped to currently implemented behavior; it deliberately does not claim unimplemented capabilities (per-callback OIDC permissions, native Deployment objects, attestations, canary or blue-green).
• Sidebar entry added for /comparison/.