Add a section on using AWS KMS keys with signed pipelines

[wolfeidau]

We have introduced the option to use AWS KMS keys with signed pipelines, this is seperate from the self managed keys already supported so I have added some setup documentation.

[buildkite-docs-bot]

Preview URL: https://2981--bk-docs-preview.netlify.app

[gilesgas]

Just a couple of things to be aware of to please the linters:

• Our public contributors guide has high-level details on how the linting works and what is linted. You can even run the linter locally to check if the build will fail before pushing up to GitHub.
• Headings should all be in sentence case. If the linter still won't pass (for example, an unusual acronym like KMS), add this alphabetically (ignoring case) into the relevant heading linter file.
• We also have guidelines on how lists should be marked up in Markdown.

[wolfeidau]

@gilesgas Thanks for the links, I most certainly need to learn how to do more complete PRs to this project. 😅