documentation site: Hugo site, milestone pages and pipeline

Milestone "Documentation site", the docs half. Siblings: agents agents!2 (merged), platform platform!1 (merged).

Issues: #3 (closed) Hugo skeleton and hand-written theme rendering the ADRs (ADRs gain frontmatter, folders are mounted so the repo layout is unchanged); #4 (closed) milestone page from a data file and a narrative, delivered-by box on the ADR page, archive list, and the amendment of ADR 0003 (order from the Depends on section, not from issue links); #5 (closed) pipeline: build on MR, build and rclone sync to R2 on main; #6 (closed) the narratives and data of the three existing milestones.

Not in this MR: #7 (closed) verification and #8 (closed) runbook. They need the site live, so they follow the apply in platform and the merge of this MR, as a second MR on the same milestone.

Merge after agents (the pipeline includes agents' ci/) and after platform is applied and merged (the publish job needs the CI variables). The MR pipeline only builds; the first main pipeline publishes.