[8.x] Add 'Docker and Kubernetes' to Deployment Models page (backport #1334)

[mergify]

This adds a "Docker and Kubernetes" section to the Deployment models page, which provides links to the various ways of configuring Elastic Agents to work with Elasticsearch, via Fleet Server.

The links point to:

• Run Elastic Agent in a container
• Run Elastic Agent on Kubernetes managed by Fleet

Both pages have been updated to include Fleet Server setup steps, via #1184

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DOCS UPDATE

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This is an automatic backport of pull request #1334 done by [Mergify](https://mergify.com).

[github-actions]

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