Metadata for every page

[gingerwizard]

Every page should have a metadata block at the top - unless a snippet.

This will aid automatic translations - modifications and incorrect formatting of this block is the most common issue.

should consist of:

---
slug: '/page/a_page'
title: 'a page'
description: 'A page'
---

Rules:

• no preceeding whitespace
• new line after last ---
• title, slug and description mandatory
• description used for search snippet.
• sidebar_label and keywords optional (must be a list)
• use single quotes on all values (mandatory as needed for some langs)
• Single space between yaml key and value

This will be checked at build time on en and all translations.