[docs][icons] Fix Font Awesome Chip demo in dark mode

[siriwatknp]

Problem

The second Font Awesome demo on the icons page (the one showing Chip + Icon) defines a nested theme via createTheme with MuiIcon styleOverrides. Without color schemes declared, that nested theme does not generate the dark-mode style layer, so the overrides don't apply when the docs are viewed in dark mode.

Fix

Add colorSchemes: { light: true, dark: true } to the demo's createTheme call so the generated styles apply in both modes.

 const theme = createTheme({
+  colorSchemes: { light: true, dark: true },
   components: {
     MuiIcon: { ... },
   },
 });

Applied to both FontAwesomeIconSize.tsx and FontAwesomeIconSize.js.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-48576--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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