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>

docs/plugins/prometheus.md

@@ -73,6 +73,8 @@ Open one to see its YAML beside a **preview**: the template drawn over a target
 - **Duplicate**: copy any template, built-in ones included, under a new name, and change the copy. Built-in templates are read-only: they ship with Steward and change with it.
 - **Export**: download a template's YAML, to keep or to share.
 
+How to write one, every block, and the format reference: [Prometheus widget templates](prometheus-widgets.md).
+
 Saving needs a template that passes the checker and a name no other template has. A widget remembers which template it draws with, so renaming a template keeps its widgets; deleting one leaves its widgets saying the template is gone. Templates are kept in Steward's backups, and adding, changing or deleting one is in the audit log.
 
 ## Alerting on a target