docs: clarify external scope in configuration reference

[joshua-temple]

Problem

The external: section heading and one-line description left it ambiguous whether the field covers GitOps mirror patterns (pushing rendered manifests to a separate repo and tracking the pushed commit) or the satellite/sibling-repo artifact coordination pattern. Issue #2 calls this out.

Fix

Added a concise paragraph immediately under the section heading in docs/src/content/docs/configuration.md that states plainly:

• what external: is for: satellite repos that own their own builds, deploy to their first environment, and notify the primary via workflow_dispatch; the primary records SHA/version and drives promotions
• what it is not for: GitOps mirror deploys (push-manifests-to-config-repo); those belong in a callback workflow, with a link to the Deploy Workflow Contract in callback-contract.md

Verification

• Guardrail scan on diff: clean (forbidden strings and em dashes checked)
• Docs build: Node 22 not available in this environment; Starlight build not run locally
• Commit is GPG-signed and DCO signed-off

Refs #2