docs(agentos): add embedded quickstart

[NathanFlurry]

• Add separate actor and embedded quickstarts, with the embedded VM reference under Reference > More.
• Make Embedded API guidance example-first across capability pages, backed by checked source files for software, persistence, credentials, agent-to-agent calls, Browserbase, and other core APIs.
• Identify actor-only hosting features without presenting misleading embedded equivalents.
• Keep the embedded reference focused on lifecycle, persistence, configuration, sidecar behavior, and capability links.
• Remove the redundant Crash Course docs page and use @rivet-dev/agentos-core throughout actor-free examples.
• Validate formatting, frozen lockfile consistency, snippet paths and regions, local embedded anchors, and comparison-table parity; rendered crawling and full typechecking remain limited by missing generated website/runtime outputs.

[railway-app]

🚅 Environment agentos-pr-1930 in rivet-frontend has no services deployed.