docs(integrations): add Model Armor integration page

[saurabh4269]

Partially addresses #2179

Summary

Adds docs/integrations/model-armor.md adapted from the adk-python integration guide (ADK 2.8.0). Updates the Model Armor bullet on docs/safety/index.md to link to the new page.

Files

• docs/integrations/model-armor.md (new)
• docs/safety/index.md (link to integration page)
• docs/integrations/assets/model-armor.png (placeholder catalog icon; happy to replace with an official Model Armor asset if maintainers point to one)

Testing

• Relative links and code blocks reviewed against adk-python guide on main
• mkdocs serve locally: page renders at /integrations/model-armor/ (not run in this environment)

Note on icon

No model-armor asset existed in docs/integrations/assets/ on main. This PR adds a temporary placeholder so the catalog frontmatter resolves. I can swap in an official GCP asset in a follow-up commit if reviewers provide the preferred file.