Move SKILL.md to .github/skills/translation/ and conform to agentskills.io specification (doxygen#12136)

* Add SKILL.md: translation agent instructions for doxygen/doxywizard i18n

Agent-Logs-Url: https://github.com/doxygen/doxygen/sessions/6828eda6-d36e-4244-a904-4286ecd71b01

Co-authored-by: doxygen <1300762+doxygen@users.noreply.github.com>

* SKILL.md: clarify git diff placeholders and h-vs-ts encoding distinction

Agent-Logs-Url: https://github.com/doxygen/doxygen/sessions/6828eda6-d36e-4244-a904-4286ecd71b01

Co-authored-by: doxygen <1300762+doxygen@users.noreply.github.com>

* Move SKILL.md to .github/skills/translation/ per GitHub agent skills convention

Agent-Logs-Url: https://github.com/doxygen/doxygen/sessions/3fb300a7-cc65-493d-9927-40239471c6ec

Co-authored-by: doxygen <1300762+doxygen@users.noreply.github.com>

* Add agentskills.io-compliant YAML frontmatter to SKILL.md

Agent-Logs-Url: https://github.com/doxygen/doxygen/sessions/6acc4a0b-84a5-4b60-9e30-3030eb0268a9

Co-authored-by: doxygen <1300762+doxygen@users.noreply.github.com>

* Update translation mechanisms and related documentation

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: doxygen <1300762+doxygen@users.noreply.github.com>
Co-authored-by: Dimitri van Heesch <doxygen@gmail.com>

Author: Copilot