Add a new Guide: Azure Cloud extensions in Marketplace

Create a new guide in the docs/_guides folder, using the same format as **_template.md**

Title: Azure Cloud extensions in Marketplace
Content: 

Add front matter to this guide, something like this:

```
---
title: Azure Cloud extensions in Marketplace
layout: default
category: ESDC: In The Cloud
summary: Add a short description of the guide here.
date: YYYY-MM-DD
---
```

Add further content here
