Add maintainer runbook for the major-version cutover

[miharp]

What

Adds MAINTAINING.md — a maintainer runbook for the major-version cutover, so
there's a concrete, shareable procedure for when OpenVox 9 (or any product's new
major) ships.

It covers:

• How versions are wired — collections + the _<product>_latest symlink,
  _data/products.yml as the registry, nav files, the version selector, and pinned
  reference generation.
• Phase 1 — stand up the new version (preview): copy the collection, sweep the
  authored pages for version strings (git grep, so generated pages are excluded),
  register the collection in _config.yml, wire nav, add a products.yml version
  block pinned to the new tag, generate + verify. latest stays on the current major.
• Phase 2 — promote to latest (GA): repoint the symlink, move the latest
  collection's nav to the new version, set latest:, freeze the old version's pin,
  and run the no-redirect page-set diff.
• Rollback.

Validation

Validated end to end in a throwaway sandbox combined with the version selector
(#335) — a full OpenVox 9.x cutover of both phases, built against the pinned
9.0.0-alpha1 references.

Phase 1 — stand up 9.x alongside (preview)

$ cp -r docs/_openvox_8x docs/_openvox_9x
$ cp _data/nav/openvox_8x.yml _data/nav/openvox_9x.yml
$ git add docs/_openvox_9x _data/nav/openvox_9x.yml

$ git grep -nE 'OpenVox 8|8\.x' -- docs/_openvox_9x _data/nav/openvox_9x.yml | wc -l
25            # reviewed: mechanical hits bumped to 9; 11 version-specific 8.x items left

# register the collection in _config.yml + _data/nav_map.yml + _data/navigation.yml, add the products.yml pin
$ bundle exec rake references:openvox VERSION=9.0.0-alpha1 COLLECTION=_openvox_9x INSTALLPATH=docs
$ bundle exec jekyll build
            done in 33.9 seconds.

Phase 2 — promote 9.x to latest (GA)

$ ln -sfn _openvox_9x docs/_openvox_latest
# openvox_latest nav -> openvox_9x; move openvox_latest into the 9x nav_map entry; products.yml latest: 9x
$ bundle exec jekyll build
            done in 33.7 seconds.

$ for v in 8.x 9.x latest; do curl -s -o /dev/null -w '%{http_code} ' .../openvox/$v/; curl -s .../openvox/$v/ | grep -o '<title>[^<]*'; done
  /openvox/8.x/    200  <title>OpenVox 8 | OpenVox Documentation   (frozen)
  /openvox/9.x/    200  <title>OpenVox 9 | OpenVox Documentation
  /openvox/latest/ 200  <title>OpenVox 9 | OpenVox Documentation   (flipped to 9.x)

Result: /openvox/latest/ serves OpenVox 9, the version picker badges 9.x as
latest, and 8.x stays frozen. The version-string sweep leaves only legitimate
version-specific 8.x content (the release-notes history and hiera backward-compat
notes).

Part of #325.

Note: the older WORKFLOW.md / WORKFLOW_reference.md predate the current
docs/_<collection>/ structure and task names; this runbook documents the current
setup. Reconciling/retiring those is out of scope here.