Add documentation for Code Security configuration file

[jasonforal]

What does this PR do? What is the motivation?

This PR updates the Code Security SAST documentation to detail the newer code-security.datadog.yaml schema/format (https://github.com/DataDog/schema/tree/main/code-security).

Details:

• This update has backward compatibility. There is no need to update non-English documentation.
• We will be performing a migration on behalf of users. We're thus not providing a "Migration Guide". I've left a small reference to the existing schema in its own section ("Legacy configuration").

Merge instructions

Merge readiness:

• Ready for merge Please do not merge yet. Our team needs to release a UI update first and will coordinate with internal communications.

AI assistance

The only LLM usage was to generate the Markdown tables for the schema keys/values and descriptions. It based this off our official JSON Schema.

Additional notes

• The link to https://github.com/DataDog/datadog-static-analyzer/blob/main/doc/legacy_config.md will function properly after merging Add documentation for code-security.datadog.yaml configuration file datadog-static-analyzer#852 (which will be done alongside merging into this repo)

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/jason.foral/K9VULN-11777/ide_plugins/vscode/
• https://docs-staging.datadoghq.com/jason.foral/K9VULN-11777/security/code_security/static_analysis/custom_rules/tutorial
• https://docs-staging.datadoghq.com/jason.foral/K9VULN-11777/security/code_security/static_analysis/github_actions
• https://docs-staging.datadoghq.com/jason.foral/K9VULN-11777/security/code_security/static_analysis/setup/
• https://docs-staging.datadoghq.com/jason.foral/K9VULN-11777/security/code_security/troubleshooting/

[maycmlee]

Create this docs card for review: https://datadoghq.atlassian.net/browse/DOCS-13719