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

[allen0099]

0.3.8 adds a runnable examples/ directory with a test that exercises each app. The docs still carry their own copies of the same code, so the two can drift apart again.

Proposal

• Enable pymdownx.snippets in both zensical.toml and zensical.zh-TW.toml, with check_paths = true so a missing file fails the strict build.
• Replace the full-app examples in the guides with --8<-- includes of the matching examples/*.py (whole file or named sections), in EN and zh-TW.
• Short fragments that only illustrate one call may stay inline.
• Keep comments inside the example files in English; the zh-TW pages explain the code around the include.

Done when

• Every complete app shown in the docs comes from a file under examples/.
• Both strict docs builds pass, and tests/test_examples.py covers every included file.