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/templates/prometheus/template.html

@@ -36,7 +36,7 @@
     <label class="field-label" for="tpl-text">YAML</label>
     <textarea class="input mono tpl-text" id="tpl-text" name="text" spellcheck="false" wrap="off"
               {%- if not editable %} readonly{% endif %}
-              placeholder="Paste a template here, or write one: name, version and blocks. The guide is docs/plugins/prometheus.md.">{{ text }}</textarea>
+              placeholder="Paste a template here, or write one: name, version and blocks. The guide is docs/plugins/prometheus-widgets.md.">{{ text }}</textarea>
     {% if editable %}
     <div class="form-actions">
       <button type="submit" class="btn">{{ "Save" if entry else "Save template" }}</button>