documentation site: runbook of the publish and the daily sync

Closes the "Documentation site" milestone: the last two issues, both phase 3.

• #7 (closed), verification: done without code. The checklist is ticked with the date in a comment on the issue (https: 200 over HTTP/2 with a fingerprinted stylesheet; dark and phone width verified in the shipped CSS; delivered-by boxes on ADR 0002 and 0003, none on 0001 since no milestone declares it yet; next and path on the three milestone pages; the archive section is in place and empty until a milestone closes; three sync MRs merged today with the page showing the new time, 23:28 UTC). Every finding of the run was already filed and fixed (#9 (closed), #10 (closed), #11 (closed), #12 (closed), platform#11 (closed)). The issue is closed with the record as its last comment.
• #8 (closed), runbook: runbooks/documentation-site.md, from the tested behaviour: pipeline, bucket and hostname, Cloudflare token permissions, the daily sync from a clean checkout to a merged MR, writing a narrative, the drift findings and what to do with each, and where to look when something breaks. The README lists it.

Nothing is left to the operator beyond the merge, which publishes the runbook and shows the runbooks link in the nav. Once merged and the milestone closed, the next sync will show the archive section of /milestones/ for the first time, which completes the one checklist item that could not render today.