CI: Add Gerrit RTDv3 merge workflow

[ModeSevenIndustrialSolutions]

Summary

Adds an organization-level required merge workflow that publishes Read
the Docs (RTDv3) documentation once a Gerrit change merges.

Part of the effort to modernise the Linux Foundation Gerrit CI and
standardise on the centrally-managed reusable workflows in
lfit/releng-reusable-workflows.

Design

Thin dispatcher (on: workflow_dispatch, invoked by the Gerrit → GitHub
integration on merge):

1. Notify merge start — posts a start comment to Gerrit (comment-only,
   the change has already merged).
2. Read the Docs merge — delegates the publish to the shared reusable
   gerrit-compose-required-rtdv3-merge.yaml@v0.8.1, forwarding the RTD
   API token.
3. Report merge status — reports the aggregated conclusion back to the
   change as a comment.

Notes / follow-ups

• All uses: references are SHA-pinned to their latest releases.
• Passes zizmor --persona=auditor with zero findings. The
  secrets-outside-env allow-list is provided by the companion config PR
  (CI: Add zizmor auditor configuration #45), which should merge first.
• Org provisioning required (admin): relies on LFIT_GERRIT_SERVER,
  LFIT_GERRIT_KNOWN_HOSTS, LFIT_GERRIT_SSH_REQUIRED_USER /
  LFIT_GERRIT_SSH_REQUIRED_PRIVKEY, the RTD_TOKEN secret, and the shared
  reusable's vars.GERRIT_URL. A repository Ruleset must wire this
  workflow to run on merge for the Gerrit-mirrored docs repositories.
• Enable this only if LF wants docs published via RTDv3 (companion to the
  bypassable verify workflow in CI: Add Gerrit bypassable RTDv3 verify workflow #47).

Validation

• actionlint → clean
• yamllint → clean
• zizmor --persona=auditor --offline → no findings