[Onboarding] Security quickstart guides

[jmikell821]

Contributes to https://github.com/elastic/docs-projects/issues/513.

Previews:

• SIEM guide
• Endpoint Security guide
• Cloud security posture management guide

[github-actions]

🔍 Preview links for changed docs

• solutions/security/get-started/get-started-cloud-security.md
• solutions/security/get-started/get-started-detect-with-siem.md
• solutions/security/get-started/get-started-endpoint-security.md
• solutions/security/get-started/quickstarts.md

[theletterf]

I'm loving it! One suggestion: I would put the substeps inside a dropdown directive to reduce cognitive load and make the procedure look leaner.

[natasha-moore-elastic]

Left some minor, mostly formatting suggestions – overall looks great!

[approksiu]

Looks like a great start! Added suggestions for the rules section.

[nastasha-solomon]

Part of me feels like this might be more appropriate under "Next steps" since all of the initial environment setup is complete. I also noticed that event analyzer is mentioned in step 3 here and later under "Next steps" here. If you decide to keep step 3 under the "Prerequisites" section, maybe remove the event analyzer mention from "Next steps"?