[Change Proposal] Compatibility with next Elastic docs

**Proposal**: make packages/integrations compatible within the next Elastic docs system.

---

The next docs system uses [MDX](https://mdxjs.com), an extension of markdown that supports jsx.

We need to update templates / generators as required to support MDX, and therefore be consumable by the system.

Integrations is a solid candidate for our "MVP" of this system, and we can make major steps once these files are supported.

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**Requirements**

1. Add frontmatter so that the pages can be interpreted and organized within the next system.

ie.

```
---
id: awsIntegration
slug: /integrations/aws
title: AWS Integration
image: An image?
summary: Summary lalala.
tags: ['integrations']
related: ['fleet', 'observability']
date: 2077-10-13
---

# AWS Integration

{content}

```

2. Apply the `.mdx` extension to READMEs instead of `.md`.
3. Update documentation for future contributors

It is understood that these files, or a version of these files, will be consumed by Kibana for use within the integration flow.

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**Related links**

* [Next doc repository](https://github.com/elastic/elastic-docs-next)
* https://github.com/elastic/observability-docs/issues/509