[DOCS-13412] Add LLM Observability and Secret Scanning to SDS docs

[PronoyC]

What does this PR do? What is the motivation?

Fixes DOCS-13412

Customers are confused about whether Sensitive Data Scanner supports LLM Observability and Secret Scanning. Both products are GA and fully integrated with SDS, but the SDS documentation doesn't mention them, creating a one-way reference problem where customers can't discover these integrations from the SDS docs.

This PR adds documentation for both integrations:

Changes to _index.md:

• Updated overview paragraph to mention LLM Observability traces and code repositories
• Added new "## Scan LLM Observability data" section explaining the managed configuration model
• Added new "## Scan code repositories" section for Secret Scanning
• Added reference links for LLM Observability and Secret Scanning

Changes to setup/_index.md:

• Added LLM Observability bullet point with link to Settings page
• Added Secret Scanning bullet point with link to documentation
• Added corresponding reference links

Expected impact:

• Enables bidirectional discovery between SDS, LLM Observability, and Secret Scanning
• Reduces "does SDS support X?" support questions
• Documents all 4 supported data sources (previously only 2 were documented in SDS docs)

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

• Follows existing documentation patterns (parallel "## Scan X" sections)
• Only additive changes, no breaking changes
• Pure documentation gap fill for existing GA features

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/pronoy/docs-13412-llm-obs-secret-scanning/security/sensitive_data_scanner/
• https://docs-staging.datadoghq.com/pronoy/docs-13412-llm-obs-secret-scanning/security/sensitive_data_scanner/setup/