docs: clickable Key links, footer source link, move operator material to co-op/ops-docs

- README: render the Key links as real Markdown links (mailto: for the
  contact address); drop the 'lives in a public git repository' note now
  that the footer carries the source link.
- mkdocs.yml: add a theme override (custom_dir: overrides) that puts a
  'Documentation source' link in the footer of every page.
- Move the operator runbook, maintenance notes and incident write-up out
  of the published site into the git-only co-op/ops-docs repository;
  remove them from the nav and repoint cross-references at Gitea.

infrastructure.md

@@ -299,9 +299,16 @@ through it; the other providers are called directly.
 
 Maintaining the co-op's infrastructure — rotating Cloudron tokens, editing the
 portal's environment, and troubleshooting the provisioning pipeline — is
-documented in the [operator runbook](operator-token-runbook.md).
+documented in the [operator
+runbook](https://git.inference.coop/co-op/ops-docs/src/branch/main/operator-token-runbook.md).
+All operator-facing material (that runbook, the maintenance notes, and incident
+write-ups) lives in the git-only
+[`co-op/ops-docs`](https://git.inference.coop/co-op/ops-docs) repository, which
+is deliberately not published on this site.
 
 ## Incident log
 
-Write-ups for outages live in [`incidents/`](incidents/). One so far:
-[2026-10-02 — gateway outage after a security-upgrade rebuild](incidents/2026-10-02-gateway-cve-upgrade.md).
+Write-ups for outages live in the operator docs, in
+[`incidents/`](https://git.inference.coop/co-op/ops-docs/src/branch/main/incidents).
+One so far:
+[2026-10-02 — gateway outage after a security-upgrade rebuild](https://git.inference.coop/co-op/ops-docs/src/branch/main/incidents/2026-10-02-gateway-cve-upgrade.md).