fix(prometheus): the list page is a section top; the scrape test builds its text without format(); docs (#5668, #5670)

- tests/test_breadcrumbs.py: the Prometheus target list is its section's
  own top page, like SNMP's, so it carries no trail.
- The integration test's exposition has label braces, which str.format read
  as fields: the fake scrape raised, the target was recorded down and no
  series landed. It substitutes the counter with replace() now.
- docs/plugins/prometheus.md: adding a target, filtering, how series are
  named and stored, alerting, and the Fabled Archives recipe.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_breadcrumbs.py

@@ -21,6 +21,7 @@ TOP_LEVEL = {
     "steward/templates/account/index.html",
     "plugins/docker/templates/docker/index.html",
     "plugins/snmp/templates/snmp/index.html",
+    "plugins/prometheus/templates/prometheus/index.html",
     "plugins/traefik/templates/traefik/index.html",
     "plugins/unifi/templates/unifi/index.html",
     # Logged out, or no page to go back up from.