docs(examples): add dockerized knowledge base example

[david-debest]

Closes #37

Summary

Adds examples/12-dockerized/, showing how to package a knowledge base as a Docker image and run it next to a Smart Connector and Knowledge Directory with Docker Compose.

• app.py: a minimal ANSWER KB, started with knowledge-mapper run app.py:kb. The CLI unregisters the KB on SIGTERM, so docker stop shuts it down cleanly.
• asker.py: a one-shot ASK KB, run from the same image, that queries app.py once to show an end-to-end interaction and then exits.
• Dockerfile: multi-stage build. uv installs the locked dependencies into /opt/venv, and a slim runtime image runs the app as a non-root user. knowledge-mapper is pinned to the PyPI release (0.1.0rc1) through the example's own pyproject.toml/uv.lock, like a real user project would.
• compose.yaml: Knowledge Directory, Smart Connector, the answer KB and the asker.
• KB identity and the SC endpoint come from environment variables via KnowledgeBaseSettings (KNOWLEDGE_BASE__ID, KNOWLEDGE_ENGINE_ENDPOINT, …), so the image contains no configuration.
• README.md, examples/README.md and CONTEXT.md now list the example.

The issue also asked to remove the root Dockerfile. That was already done before this PR (and examples/legacy/ is gone too). The issue suggested the name 10-dockerized, but 10- is taken, so this is 12-.

Testing

• docker compose up --build: the asker receives both answers and exits 0. The answer KB keeps running.
• docker compose stop answer-kb: the KB unregisters and exits 0.
• If the SC is unreachable, knowledge-mapper run exits non-zero, so restart: unless-stopped retries it.
• ruff check / ruff format --check pass.