docs(chart): slim README 699->298, link to canonical docs

[trilamsr]

Summary

Slimmed install/kubernetes/tracecore/README.md from 699 → 298 lines
(-401 LoC, -57%). The chart README had grown into a third operator
manual, duplicating canonical docs the repo already maintains.

What was cut

• Per-knob values essays — the 56-row narrative values table is now
  a top-level group pointer. Authoritative per-knob commentary already
  lives in values.yaml (post-Lane-L trim).
• Recipe walkthroughs (DCGM scrape config, OTLP exporter field
  reference, vanilla Prometheus pod-SD job, Loki/Tempo/Datadog hints,
  journald details) → pointers to docs/integrations/.
• Operator-incident playbook prose in Troubleshooting (verbose
  rollout-math paragraphs, line-by-line probe arithmetic, kubelet probe
  state-machine narrative) → pointer to docs/RUNBOOK-tracecore.md,
  bullets only for chart-install gotchas.
• PSS compliance table collapsed into a single-paragraph defaults
  summary; conftest + CI remain the load-bearing enforcement.
• Production-preset prose trimmed to bullet rationale + canonical
  apply snippet.

What was kept (load-bearing)

• All install/upgrade/uninstall commands.
• All anchors inbound refs depend on: #troubleshooting,
  #values-reference, #common-configurations, and the section
  headings "Install", "Upgrade", "Upgrade posture", "Production
  preset", "Pod Security Standard compliance", "Documented deviations".
• Canonical-doc pointers up top so first-time readers route immediately.

Cross-refs preserved

Pre-flight grep verified these inbound refs still resolve:

• docs/getting-started.md — file-level ref, unchanged.
• docs/reference-architectures/01-single-cluster.md —
  #troubleshooting, #values-reference, #common-configurations
  anchors preserved.
• docs/reference-architectures/README.md — file-level ref, unchanged.
• install/kubernetes/tracecore/values-production.yaml — §"Production
  preset" heading preserved.
• install/kubernetes/tracecore/templates/networkpolicy.yaml —
  §"NetworkPolicy" content covered under #common-configurations.
• docs/rfcs/0008-auto-update-boundary.md — §"Upgrade" / §"Upgrade
  posture" preserved.
• docs/integrations/cert-manager-mtls.md — file-level ref, unchanged.

(Pre-existing line-number-anchored refs in
docs/v1-rc1-operational-gaps.md describing the README as stale will
need a separate sweep — they document drift this PR partly addresses.)

Test plan

• wc -l install/kubernetes/tracecore/README.md → 298 (< 300).
• make doc-check green pre + post (1115 markdown links + 218
  anchor slugs resolve).
• All inbound refs grep-verified to still resolve.

docs(chart): slim install/kubernetes/tracecore/README.md from 699 to 298
lines. Recipe walkthroughs moved to docs/integrations/, operator
incident playbook to docs/RUNBOOK-tracecore.md, per-knob essays
deferred to values.yaml. All anchored cross-refs preserved.