Add noindex pages, robots.txt and operator footer

- Page.noindex: marked pages get a robots meta tag and a Disallow line in the
  generated robots.txt (single source of truth in src/pages.py)
- templates/robots.txt: public pages indexable, noindex pages and data files not
- Footer shows an optional operator line from contents/snippets/footer_operator.md
- Tests for robots.txt, noindex meta and footer

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

templates/robots.txt

@@ -0,0 +1,11 @@
+# robots.txt – generated by the site generator, do not edit by hand.
+# The course pages may be crawled and indexed. Pages with contact details may not.
+
+User-agent: *
+Allow: /
+{% for page in noindex_pages -%}
+Disallow: /{{ page }}
+{% endfor -%}
+# Data files (search indexes etc.) are not pages
+Disallow: /*.json$
+Disallow: /*.zip$