[docs] Isolate error messages, e.g. MUI -> Material UI

[oliviertassinari]

Steps to reproduce

We prefix all the errors with MUI: but we shouldn't, we should use the project name instead.

Current behavior

This error is about MUI System, it's not about MUI

material-ui/packages/zero-runtime/src/utils/cssFunctionTransformerPlugin.ts

Lines 54 to 57 in 9e0af1b

	if (!themeValue) {
	console.warn(
	`MUI: Value for key: ${val} does not exist in "theme.${propertyThemeKey}" or "theme.vars.${propertyThemeKey}"`,
	);

Expected behavior

 if (!themeValue) { 
   console.warn( 
     `MUI System: Value for key: ${val} does not exist in "theme.${propertyThemeKey}" or "theme.vars.${propertyThemeKey}"`, 
   ); 

Context

This is a continuation of mui/mui-x#11645.

For the back story, we are moving toward forcing developers to import from @mui/base, @mui/system, and @mui/material directly (no longer reexporting anything from @mui/material). This is a strategic move to:

• better isolate the responsibilities, to educate developers that each of these 3 modules solves different problems
• better isolate the bundle size, removing the perception of bloat
• better support the progressive adoption of the MUI stack