Add Leostream Connection Broker integration page

[jasnoyaeger]

Summary

• Adds a new product guide page at docs/product-guide/tools-integrations/leostream.md documenting Leostream as a supported VDI connection broker for VergeOS.
• Covers the vendor-neutral broker model, supported display protocols, mixed-platform brokering, recipe-based provisioning with payload (Leostream 2023.2), the Leostream component architecture (Connection Broker, Gateway, Agent, Connect client) running as VMs on VergeOS, and compliance positioning (HIPAA, CUI).
• Documents configuration via a dedicated VergeOS user account (Leostream authenticates against VergeOS with username and password) and cross-links to the NVIDIA GPU Virtualization page for vGPU/MIG-enabled desktops.
• Adds the page to mkdocs.yml under VDI & Desktop Delivery alongside the existing Inuvika entry.

Closes #457

Test plan

• Verify the page renders on mkdocs serve and appears under VDI & Desktop Delivery in the left nav
• Confirm external links resolve: Leostream Quick Starts & Guides, Leostream + VergeIO partner page, Verge.io press release, Leostream Support portal
• Confirm internal cross-link to NVIDIA GPU Virtualization resolves
• CI: frontmatter validation, internal link checks, build passes

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