feat: Auto-sync navigation from 2.6.x docs.json

[bokelley]

Summary

Updates the sync-versioned-docs workflow to automatically sync navigation config from 2.6.x's docs.json to main's v2.6-rc version.

Problem

When new sections (like governance) are added on 2.6.x, the docs files get synced but the navigation config in main's docs.json had to be manually updated - causing missing sidebar entries.

Solution

The workflow now:

1. Extracts navigation groups from 2.6.x's docs.json (the "2.5" version which has the actual content)
2. Transforms all paths by prepending v2.6-rc/
3. Updates main's docs.json with the transformed navigation for the 2.6-rc version

Dependencies

• PR fix: Convert governance docs to relative links #820 fixes broken relative links in governance docs (should merge first)

Test plan

• jq transformation tested locally
• YAML validation passes
• Run workflow manually after merge to verify

🤖 Generated with Claude Code