[DOCS-8805] restructure of Getting started

[aliciascott]

What does this PR do? What is the motivation?

Phase 1:

• alphabetizing the list (with the exception of Learning Center and Support)
• moving all the Security Getting started pages under a new Security heading

Merge instructions

Merge readiness:

• Ready for merge

For Datadog employees:
Merge queue is enabled in this repo. Your branch name MUST follow the <name>/<description> convention and include the forward slash (/). Without this format, your pull request will not pass in CI, the GitLab pipeline will not run, and you won't get a branch preview. Getting a branch preview makes it easier for us to check any issues with your PR, such as broken links.

If your branch doesn't follow this format, rename it or create a new branch and PR.

To have your PR automatically merged after it receives the required reviews, add the following PR comment:

/merge

Additional notes

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/aliciascott/getting-started-phase1/getting_started/security/

Renamed files

• https://docs-staging.datadoghq.com/aliciascott/getting-started-phase1/getting_started/security/application_security
• https://docs-staging.datadoghq.com/aliciascott/getting-started-phase1/getting_started/security/cloud_security_management
• https://docs-staging.datadoghq.com/aliciascott/getting-started-phase1/getting_started/security/cloud_siem