docs: add concise beta deployment instructions

[dadachann]

Summary

• Add short opt-in beta Docker and uvx instructions in zh/en/ja; leave all existing stable instructions unchanged.
• Download dev/4.11.x/docker/docker-compose.yaml as compose.yaml, manually set every shared LangBot image (Core/plugin runtime/Box) to rockchin/langbot:beta, and pull/start with the existing all profile.
• Pin uvx to langbot==4.11.0b3 with --prerelease=allow --refresh, including prerelease dependencies.

Validation

All local install/test/build commands were limited to two CPUs (taskset -c 0,1) and ran as chan/dadachann using Node 24.

• BETA_DIAGNOSTICS_STATIC=1 npm test: 36 Node tests + 20 Python tests passed, no skips.
• npm run typecheck: passed.
• npm run build: passed, including 31 static-output tests and a new rendered-article contract for all six changed pages; 305 MDX documents / 482 routes. No renderer errors.
• Source deployment contracts cover stable preservation, beta template URL, every image service and exact uvx options.
• Compose URL returned HTTP 200 and matched the current isolated Core template; docker compose --profile all -f - config --format json verified all three manually changed images resolve to rockchin/langbot:beta without starting containers.

Publication

Do not merge until the parent release review is ready. Beta 3 PyPI packages/dependencies and the Docker beta alias are published separately by the release owner. No Docker/PyPI/GitHub release was published here. Public docs also require the landing docs-submodule update and normal site build/deployment after merge; no landing source changed.