[Containers] Document scheduling policies and snapshots - #33531

Summary

• Builds on Add docs and changelog for containers snapshots #31946 and preserves the original snapshot commit and authorship.
• Documents the new durable_object scheduling policy alongside the existing default policy.
• Covers named images, runtime instance sizing, scheduling-policy immutability, snapshot restore constraints, and rollout behavior.
• Updates the Containers architecture and configuration pages, Durable Object Container API, and Wrangler configuration reference.
• Adds a Containers changelog entry for the scheduling-policy public beta.

Validation

• pnpm run check (Astro: 0 errors, warnings, or hints; Worker TypeScript passed)
• git diff --check
• Reviewed against the Cloudflare documentation style guide and adjacent Containers docs
• Full local build reached the bundling stage, then stopped because the local disk filled; GitHub CI will run the full build.

Documentation checklist

• Includes a changelog entry.
• Adheres to the documentation style guide.