Add source code integration instructions to security docs

[drichards-87]

What does this PR do? What is the motivation?

Adds source code integration instructions to CSM setup docs.

Merge instructions

• [ ] Please merge after reviewing

Additional notes

[github-actions[bot]]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/deforest/source-code-integrations-setup/security/cloud_security_management/setup/source_code_integrations

Modified Files

• https://docs-staging.datadoghq.com/deforest/source-code-integrations-setup/security/cloud_security_management/setup/