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/template_format.py

@@ -3,7 +3,8 @@
 A template says which of a target's series a dashboard widget shows and
 how. It is data only: it names series and lays them out in blocks, and
 Steward draws them (template_view.py). It can't add a visual, a colour or a
-formula; a new block is a Steward release (System 181's ruling).
+formula; a new block is a Steward release (System 181's ruling). The
+format, for people writing one: docs/plugins/prometheus-widgets.md.
 
     name: Fabled Archives
     version: 1