docs: restructure, add cross-references to global themes documentation

[devalog]

Summary

Improvements to the global themes documentation and surrounding cross-references.

Page placement & references

• Moved global-themes.mdx from Configuration → Customization (it's a branding mechanism that operates on the same fields already
  grouped under Customization, like header/footer and css/js)
• Added a redirect from /learn/docs/configuration/global-themes → /learn/docs/customization/global-themes
• Added a global-theme entry to the Core configuration list on the site-level settings page
• Added cross-references from each fern docs theme command in the CLI reference back to the global themes page

Content cleanup on the global themes page

• Renamed the "How it works" heading to "Set up a global theme" (it's a setup walkthrough, not a conceptual overview)
• Converted the h3 step headings inside to components
• Linked the action verb in each step (Export, Upload, List, publish command) to the relevant entry in the CLI command reference
• Replaced the two flat lists in "What the theme controls" with a single Field / Owner / Description table; linked each field to its canonical reference
• Integrated the override behavior into the section intro — explicitly calls out that local edits to theme-owned fields are
  overwritten on publish

[github-actions]

🌿 Preview your docs: https://fern-preview-devalog-global-themes.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-api-reference/cli-reference/commands
• learn/docs/changelog/2026/4/27
• learn/docs/customization/global-themes
• learn/docs/configuration/site-level-settings