docs(openspec): replace setup guide with introduction page

Replace the detailed OpenSpec setup guide with a concise introduction page across all locales. The new intro-openspec.mdx focuses on introducing OpenSpec's purpose and key features, while delegating specific installation and configuration steps to the official OpenSpec repository and documentation.

This change removes setup-openspec.mdx and adds intro-openspec.mdx in:
- Base documentation (src/content/docs/related-software-installation/openspec/)
- All 26 translation locales (src/content/translations/docs/*/...)

Co-Authored-By: Hagicode <noreply@hagicode.com>
Signed-off-by: newbe36524 <newbe36524@qq.com>

Author: newbe36524