Derive documented locale counts from available locales

Problem

RenderCV currently exposes 22 built-in locales, but two documentation pages use stale hard-coded counts:

• docs/user_guide/yaml_input_structure/locale.md says 12 languages.
• docs/user_guide/how_to/use_the_ai_agent_skill.md says 20 languages.

The latter is also copied into the rendercv-skill distribution README.

Expected behavior

Expose a locale_count documentation macro derived from available_locales, use it wherever the count is shown, and update the distribution README so future locale additions cannot leave these pages stale again.

This should remain separate from #791/#792 because it concerns the broader documentation templating pipeline rather than the generated SKILL.md reference.