docs: explain source verification in schema extraction tutorial

[roli-lpci]

Closes #808.

The schema tutorial still shows an incorrect article date without explaining the error. This adds the warning requested in the issue, links the original investigation, and follows it with the strip-tags -t meta command that preserves publication-date metadata.

The explanation distinguishes a schema-conforming value from a source-supported fact and publication time from modification time. It explicitly asks readers to check the extraction after preserving metadata.

Validation:

• Ran default strip-tags and strip-tags -t meta against a deterministic HTML fixture with publication and modification meta tags plus a placeholder visible date. Default output drops both date values; -t meta retains both.
• uv run --python 3.11 --with-requirements docs/requirements.txt make -C docs html: build succeeded.
• git diff --check: passed.
• No new LLM extraction was run or presented as verified. The incorrect-date example and diagnosis come from the existing tutorial and its issue investigation.

Prepared with AI assistance.