docs: document quarto.* template variable namespace

[cderv]

Document the new quarto.* Pandoc template-variable namespace, which exposes Quarto's resolved localized strings (the merged result of _language.yml + lang: locale + language: overrides) to custom templates and template partials as $quarto.language.<key>$.

Adds:

• docs/authoring/language.qmd — "Localized Strings in Templates" section explaining the namespace, with a custom-template example and an internal-namespace callout
• docs/journals/templates.qmd — short pointer section linking back to the canonical docs from the template-authoring page
• docs/prerelease/1.10/_highlights.qmd — v1.10 highlight bullet

All new content is gated by {{< prerelease-callout 1.10 >}}, so the sections stay hidden in published docs until v1.10 ships.

Companion to quarto-dev/quarto-cli#14530

[github-actions]

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🔄 Modified Documents

• docs/authoring/language.html
• docs/journals/templates.html