Story 17.2 — Multi-cluster federation operator guide (R1 land + 17.1 cross-link)

[fupelaqu]

Story 17.2 (Epic 17 — R1 Documentation & Marketing) lands the federation operator guide MD mirror on release-r1 and wires the R1 cross-link.

Path A (extend existing 16.6 PR #124, base release-r1, head feature/16.6) — not a fresh branch.

Deltas applied to documentation/client/federation_operator_guide.md:

• Dropped the chart-internal ## For Epic 17 hand-off block (A5) — kept in the canonical chart copy only.
• Added the Story 17.1 Cross-Index JOIN matrix cross-link (../sql/joins.md) as an intro See-also, with a tracked-placeholder TODO (17.1's MD twin lands via PR Story 17.1 — Cross-Index JOIN doc (Markdown twin) #137 on the same release-r1 base).
• Verified already-applied 16.6 state: canonical-source pointer, by-name example refs, README cross-links (client + sql), no helm/softclient4es-federation path.

Re-validated the guide's §5 config-reference against the live chart softclient4es-federation/values.yaml — no drift. Dual-docs parity (web MDX twin) confirmed: section headers byte-identical.

Doc-only; no code, no sbt, no chart-template change.