Fixes a stale docs reference in docs/user-guide/choosing-your-theme.md for hljs_style styles.

What changed

• Replaced [styles]: https://highlightjs.org/static/demo/ with [styles]: https://highlightjs.org/examples

Why

• https://highlightjs.org/static/demo/ now redirects to https://highlightjs.org/not-a-cdn, which no longer shows the style gallery used by readers following the MkDocs theme docs.
• https://highlightjs.org/examples is a current highlight.js page that provides style examples.

Validation evidence

• curl -I -L https://highlightjs.org/static/demo/ resolves to https://highlightjs.org/not-a-cdn.
• curl -I https://highlightjs.org/examples returns 200.
• Built local docs successfully after installing docs requirements.

Checks run

• python -m pip install -e .
• python -m pip install -r requirements/requirements-docs.txt
• python -m mkdocs build -q

Residual uncertainty

• I verified target reachability and docs build, but did not run an external link-check pipeline in CI in this run.