docs: FontAwesome 7 upgrade documentation

[imorland]

Summary

Documentation updates accompanying the FontAwesome 5→7 upgrade in flarum/framework#4388.

• New dedicated FontAwesome page in the Management section covering all three icon sources (Local, CDN, Kit), icon class name reference table, renamed icons in FA7, and extension development notes
• Updated config.md — prose and CDN URL example updated to FA7
• Updated themes.md — version reference updated, links to new FA page
• Updated extend/i18n.md — replaced dead fortawesome.github.io URL with fontawesome.com/icons
• Updated extend/update-2_0.md — added FontAwesome 5→7 section under Backend > Dependencies explaining what changed, backward compatibility, and upgrade steps for existing installs

Test plan

• Verify new FontAwesome page renders correctly in local Docusaurus dev server
• Verify sidebar entry appears in Management section between Themes and Mail
• Check all internal links resolve (fontawesome.md, config.md, admin.md)
• Verify renamed icons table and upgrade notes are accurate

Framework PR: flarum/framework#4388

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