Revamp the documentation page for templates

Reshape the page, add metadata, and construct a procedure for
creating new templates.

doc/operations/metrics/dashboards/develop.md

-# Development guide for Metrics Dashboards templates
+---
+stage: Monitor
+group: APM
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
 
-This document explains how to develop Metrics Dashboards templates
+# Developing templates for custom dashboards **(CORE)**
 
-## Place the template file in the relevant directory
+GitLab provides a template to make it easier for you to create templates for
+[custom dashboards](index.md). Templates provide helpful guidance and
+commented-out examples you can use.
 
-All template files reside in the `lib/gitlab/metrics/templates` directory
+## Create a new dashboard template
 
-## Criteria
+To create a new dashboard template to speed the development of more dashboards:
 
-All the templates must be named with the `*.metrics-dashboard.yml` suffix.
+1. Navigate to the browser-based editor of your choice:
 
-The file must follow the [custom dashboard syntax](../../../operations/metrics/dashboards/yaml.md).
+   - *To use the repository view,* navigate to **{doc-text}** **Repository > Files**.
+   - *To use the [Web IDE](../../../user/project/web_ide/index.md),* click
+     **Web IDE** when viewing your repository.
+1. Create a template file that meets your needs, using the [custom dashboard syntax](yaml.md).
+1. Save the template file in the `lib/gitlab/metrics/templates` directory,
+   with a name matching the pattern `*.metrics-dashboard.yml`.
+1. Reload the editor you used to create the new template and ensure the template
+   is now available for use:
 
-### Make sure the new template can be selected in the UI
+   - *In the repository view,* click **{plus}** **Add to tree** and select **New file**,
+     then click **Select a template type** to see a list of available templates:
+     ![Metrics dashboard template selection](img/metrics_dashboard_template_selection_v13_3.png)
+   - *In the Web IDE,* click **Web IDE** when viewing your repository, click
+     **{doc-new}** **New file**, then click **Choose a template** to see a list of
+     available templates:
+     ![Metrics dashboard template selection WebIDE](img/metrics_dashboard_template_selection_web_ide_v13_3.png)
 
-All the templates available in `lib/gitlab/metrics/templates` are selectable in the **New File** UI from the repository view as well from the **WebIDE** New File template selector.
+## Template location and naming
 
-![Metrics dashboard template selection](img/metrics_dashboard_template_selection_v13_3.png)
-![Metrics dashboard template selection WebIDE](img/metrics_dashboard_template_selection_web_ide_v13_3.png)
+For templates to be valid and available for use, they must:
+
+1. Reside in the `lib/gitlab/metrics/templates` directory.
+1. Be named with the `*.metrics-dashboard.yml` suffix.
+1. Follow the [custom dashboard syntax](yaml.md).