chore(config): strip _comment_*/_example_* clutter now that cvs man documents these

[speriaswamy-amd]

Part 13 of 14 in a stack for AIMVT-276.
Base: #345.

Why

cvs man (parts 9-12) now generates parameter docs directly from the
pydantic schemas, so the hand-maintained _comment_*/_example_* keys in
the shipped sample configs and the duplicated parameter tables in
README_preflight_config.md are a second, driftable source of truth for
the four suites this ticket documents (rccl, health, megatron, preflight).
No code path reads these keys for behavior — they are only skipped/ignored
by the config loader — so this is a pure documentation cutover; existing
customer configs that still contain the old keys continue to work
unmodified.

What changed

• Stripped all _comment_*/_example_* keys from the four pilot suites'
  sample JSON configs: rccl_config.json, mi300_health_config.json, the
  three megatron JSONs, and preflight_config.json (including the
  node_smoke block added upstream after this ticket's fork point, now
  documented by PreflightNodeSmokeConfig in schemas.py). Manual setup
  instructions under node_smoke (_setup_comment, _setup_step_*,
  _setup_note) are preserved as-is — they're procedural, not per-field docs.
• Trimmed the now-duplicated hand-written parameter tables from
  docs/reference/configuration-files/health.rst and megatron.rst, and
  from README_preflight_config.md (the fabric_checks/node-smoke/
  reporting sections upstream had added since this ticket's fork point,
  now fully covered by cvs man preflight_checks), pointing readers at
  cvs man instead.
• Scope: only the suites registered in config_registry.py (rccl, health,
  megatron, preflight). Other suites with leftover _comment_*/_example_*
  keys (platform, ibperf, ompi, inference, jax, torchtitan, mori, anc) have
  no pydantic schema yet and are out of scope for this pilot.