docs: add Beta platform reference

[biswaroop1547]

Summary

• add a top-level Beta section for the current REST API v1 surface and platform architecture
• document authentication, Agents and versions, schedules, runs, threads, files, projects, and usage
• align product documentation with current approval, MCP, enterprise skill, Entra governance, model-routing, confidential inference, privacy, security, and SOC 2 Type I behavior
• mark Beta routes as subject to change while keeping stable product corrections on their owning pages

Validation

• npm run build
• npm run types:check
• npx --yes cspell@10.2.0 "**/*.mdx" --no-progress --no-summary --show-context
• browser-checked the home, Beta authentication, and architecture pages from the static build