docs: add Microsoft Intune-managed LLM client guide

[danehans]

Company

Not applicable; this is a documentation contribution.

Contribution summary

Adds a shared Microsoft Intune guide for managing supported LLM clients with agentgateway in Kubernetes and standalone deployments. The guide covers Intune policies, agentgateway configuration, helper scripts, etc. for detailed Codex and Claude Desktop examples.

Dependency

Merge agentgateway/agentgateway#3012 first so the linked verification and compliance artifacts are available from the default branch.

Checklist

• Reuses one source across Kubernetes and standalone documentation.
• Documents platform-specific Intune capabilities and limitations.
• Links to mode-specific HTTPS listener configuration.
• Keeps provider credentials out of endpoint-management profiles.
• References reusable macOS and Windows verification scripts.
• Documents client-specific custom-compliance discovery artifacts.
• Retains an interactive request as the final routing test.

Validation

• hugo --gc --minify
• Rendered Kubernetes latest and main guides.
• Rendered standalone latest and main guides.
• git diff --check

[danehans]

Keeping as draft until I e2e test the Claude Desktop client (blocked by IT-provisioning).