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.

overrides/partials/copyright.html

@@ -0,0 +1,17 @@
+<div class="md-copyright">
+  {% if config.copyright %}
+    <div class="md-copyright__highlight">
+      {{ config.copyright }}
+    </div>
+  {% endif %}
+  <div class="md-copyright__highlight">
+    Documentation source:
+    <a href="https://git.inference.coop/co-op/docs" target="_blank" rel="noopener">co-op/docs on Gitea</a>
+  </div>
+  {% if not config.extra.generator == false %}
+    Made with
+    <a href="https://squidfunk.github.io/mkdocs-material/" target="_blank" rel="noopener">
+      Material for MkDocs
+    </a>
+  {% endif %}
+</div>