docs: include the examples/ files instead of copying code into the guides

[allen0099]

Closes #289

The guides now pull full-app code from examples/*.py with pymdownx.snippets (already enabled with check_paths = true in both Zensical configs), so the code on the page is the code tests/test_examples.py runs. A missing file or section fails the strict build.

Converted (EN and zh-TW)

• HTTP_CACHING: intro app → examples/http_cache.py:routes; protected monitoring routes → examples/http_cache.py:admin.
• BACKENDS: "Closing a backend" → examples/redis_backend.py:lifespan.
• STATE: quick start → whole examples/oauth_state.py.
• SESSION: Basic Usage → new examples/session_api.py; Full Example (Redis) → new examples/session_redis.py.
• JWT_CLAIMS: base serializer, multi-tenant serializer, Option 1 setup and the complete application → sections of new examples/session_jwt_claims.py.

The new examples have tests in tests/test_examples.py. The JWT one is skipped without the jwt extra, and the Redis one uses the same live-server opt-in as the existing Redis example. It uses its own key prefix, and clear() only touches that namespace. Section markers are # --8<-- [start:name] / # --8<-- [end:name], documented in examples/README.md, and they do not show up in the rendered page.

Left inline

• README / zh-TW index quick start: the README is also rendered on PyPI and GitHub, where snippets do not run.
• Short fragments (one call or a parameter reference): add_routes parameters, APP_CACHE and LOCK API fragments, the atomic-primitives fragment, HTTP_CACHING fragments that build on an app defined elsewhere, JWT_CLAIMS Example 1 / Option 2 / key rotation / testing snippets, CACHE_FLOW, and the SessionManagerProxy alternative in SESSION.

Other changes

• Include fences are wrapped in <!-- fmt:off --> / <!-- fmt:on -->. Without the wrapper, ruff-format rewrites --8<-- "..." in markdown code blocks as a Python expression.
• docs.yml also runs on examples/** changes.
• DEVELOPMENT.md explains how to include an example. The zh-TW GLOSSARY notes that included code keeps English comments.

Checks

• zensical build --strict passes for EN and zh-TW. The rendered HTML contains the included code and no literal --8<-- include lines (the one deliberate escaped example in DEVELOPMENT is the only one).
• pytest: 1417 passed, including the Redis example. Coverage is 99.91%.
• pre-commit run --all-files: clean.