Update implementation doc to reflect current review workflow

Update the Documentation Guide implementation spec to reflect all the
changes made to the review process and automated workflows today:

- Five lanes (added Lane 5: Dev docs) with content-based detection
- review-workflow commit status as the merge gate, managed by the bot
- Per-individual reviewer gating via hidden state comment; drive-by
  approvals are logged but don't advance the stage
- Stage does not reset when authors push new commits mid-review;
  bot pings assigned reviewers to backcheck
- admin:advance-to-director override label for stuck Lane 1 PRs
- Docs changes from non-docs branches are auto-intercepted
- Non-docs PRs get the admin-may-merge signal from ci-build.yml
- deploy.yml path filter and force-deploy escape hatch
- GA disabled on previews via DOCUSAURUS_IS_PREVIEW env var

Adds an 'Infrastructure notes' section at the bottom as a reference
for future planning work and Claude Code sessions.

Author: AdamGohs