Add a sample post template (#430)

This is aimed to make it quicker to get started, by giving you a
template you can copy with all the relevant fields.

There is a 'Copy code' button to help you quickly copy it into a code
editor.

<img width="2068" height="4024"
alt="localhost_8080_guide_a-sample-template_"
src="https://github.com/user-attachments/assets/5f878cb7-516b-4721-908a-40da2d480bd2"
/>

---------

Co-authored-by: Ed Horsford <mail@edwardhorsford.com>

Author: frankieroberto

app/guide/a-sample-template.md

@@ -0,0 +1,53 @@
+---
+layout: page
+title: A sample post template
+description: You can use this template to get you started.
+eleventyNavigation:
+  parent: Guide
+---
+
+This template contains all the possible metadata you can add to a post, and some sample markdown for the content (see [full markdown guide](/guide/using-markdown/)).
+
+You don’t have to use this template, but it may be helpful.
+
+To use it, create a new subfolder for your post, add an `index.md` file in the subfolder and copy in template below.
+
+All metadata is optional except for the title and date. Delete any lines you do not need.
+
+```markdown { .nhsuk-code--button }
+---
+title: This is the title of the post
+description: This description will appear on index pages and when sharing on social media.
+date: 2025-12-01
+tags:
+  - add
+  - tags
+  - to help group
+  - related posts
+author:
+  - Your Name
+  - Another Name
+opengraphImage:
+  src: /your-service/2025/12/post-title/image-name.png
+  alt: Alternative text for the lead image
+---
+
+An intro paragraph which summarises the whole thing.
+
+## Heading
+
+Some content.
+
+[A link to another post](/your-service/2025/11/another-post-title/)
+
+### Subheading
+
+More content
+
+![Alt text for an image](/your-service/2025/12/post-title/image.jpg)
+
+This image has a visible caption:
+
+![Alt text for an image](nhs-logo.png "Caption for the image")
+
+```