[8.18] Add a docs page about Detection-as-code rules management

[nastasha-solomon]

Description

Provides 8.18 changes for elastic/docs-content#667. Twin PR for 9.0 and Serverless is at: elastic/docs-content#891

Preview

Detections and alerts | Manage rules as code - Added a new section to the top-level "Detection and alerts" page that briefly explains what Detections-as-Code is and how to find more info.

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[mergify]

This pull request is now in conflicts. Could you fix it @nastasha-solomon? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b issue-667-dac upstream/issue-667-dac
git merge upstream/8.x
git push upstream issue-667-dac

[nastasha-solomon]

@approksiu I think the new section would be better placed on the "Detections and alerts", which generally describes detection features (e.g., rules, alerts, notifications, etc.) that users can work with. We typically use the "Manage detection rules" page to describe functionality that's available from the Rules page in the Security app.

Let me know if you're ok with the different placement or if you'd prefer that I move it back to the "Manage detection rules" page.

[approksiu]

Could you check that the security labs rendering?

[approksiu]

Thanks!