docs: add themed applications documentation

[yukazakiri]

Summary

Add documentation for themed applications including chromium, firefox, spicetify, vscode, zed, terminals, gtk-qt, sddm, cli-tools, and vencord. Also adds themed apps category in the docs sidebar.

Motivation

Users need documentation on how to theme various applications to match the iNiR desktop environment. This PR adds comprehensive guides for theming popular applications.

Testing

1. Build the docs site locally
2. Navigate to the new themed apps documentation pages
3. Verify sidebar shows the new category
4. Check all links work correctly

Checklist

• Tested on Niri (or Hyprland if applicable)
• Tested both ii and waffle families for UI changes
• Tested material, aurora, and inir styles for ii changes
• No hardcoded values (colors, fonts, durations use design tokens)
• Config changes synced in Config.qml and defaults/config.json
• Config access uses optional chaining: Config.options?.section?.option ?? default
• IPC functions have explicit return types (: void, : string, etc.)
• Shell restarted after changes: qs kill -c ii; qs -c ii
• Logs checked for errors: qs log -c ii | tail -50
• Lazy-loaded components tested (Settings, overlays)
• No console errors or warnings

Related

Closes #
Fixes #