feat: set default document licence to CC BY 4.0

[MatMatt]

Sets the default document licence to CC BY 4.0 (Creative Commons Attribution 4.0 International).

Changes:

• _meta/includes/default.yml: license default changed from "EUPL (>= 1.2)" to "https://creativecommons.org/licenses/by/4.0/"
• _meta/includes/json-ld.html: template fallback changed from "" to "https://creativecommons.org/licenses/by/4.0/"

This creates a clean override chain: document frontmatter → default.yml → template fallback. Documents needing a different licence (e.g. third-party content, software docs) override license: in their YAML header.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Update the license default used by production

The deployment still publishes EUPL (>= 1.2): build-docs.sh lines 93–101 replaces _quarto.yml with _quarto-no-headers.yml, whose metadata-files entry points to the unchanged _meta/includes/default-no-headers.yml (line 8). Because that metadata supplies a nonempty license, the new JSON-LD fallback is never selected either. Update the live default so deployed documents actually receive CC BY 4.0.

AGENTS.md reference: AGENTS.md:L31-L33

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