docs(prometheus): writing a widget template, every block, and the format reference (#5694)

docs/plugins/prometheus-widgets.md covers:
- where templates live and how to copy one in, duplicate, export and share;
- a first template, walked through;
- each block (states with its deadlines, values, times, table, info) with an
  example;
- the format reference: keys, units, spans, limits, what the checker refuses;
- the Fabled Archives template, block by block, with what it leaves out and
  why.

Linked from the README, the plugin guide, the editor and template_format.
tests/plugins/prometheus/test_widget_guide.py checks every YAML example in the
guide against the checker, so the guide can't drift from the format.

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

plugins/prometheus/widgets/fabled_archives.yaml

@@ -1,6 +1,6 @@
 # Fabled Archives, from its /metrics on the LAN-only backup listener (:8081).
-# Written against the series Steward stores for Archives (#5693); see
-# docs/plugins/prometheus.md.
+# Written against the series Steward stores for Archives (#5693); the format
+# is docs/plugins/prometheus-widgets.md.
 #
 # Left out on purpose:
 # - fabledarchives_section_up: a section Archives can't read stops reporting