[docs] Improve the @material-ui/styles documentation

[oliviertassinari]

I have removed all the imports from @material-ui/core in the advanced section of the documentation of the styles package. It fixes the codesandboxes.

Closes #15153.

[mui-pr-bot]

Details of bundle changes.

Comparing: 8a67ecc...cddf6fe

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	347,956	347,956	89,782	89,782
@material-ui/core/Paper	0.00%	0.00%	68,404	68,404	20,053	20,053
@material-ui/core/Paper.esm	0.00%	0.00%	60,735	60,735	18,785	18,785
@material-ui/core/Popper	0.00%	0.00%	34,930	34,930	11,919	11,919
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,877	17,877	5,962	5,962
@material-ui/core/useMediaQuery	0.00%	0.00%	2,463	2,463	1,044	1,044
@material-ui/lab	0.00%	0.00%	148,083	148,083	43,917	43,917
@material-ui/styles	0.00%	0.00%	53,143	53,143	15,444	15,444
@material-ui/system	0.00%	0.00%	17,132	17,132	4,519	4,519
Button	0.00%	0.00%	88,669	88,669	26,420	26,420
Modal	0.00%	0.00%	82,688	82,688	24,802	24,802
colorManipulator	0.00%	0.00%	3,745	3,745	1,537	1,537
docs.landing	0.00%	0.00%	49,820	49,820	10,807	10,807
docs.main	+0.01% 🔺	+0.01% 🔺	644,555	644,591	200,662	200,684
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	296,610	296,610	82,782	82,782

Generated by 🚫 dangerJS against cddf6fe

[joshwooding]

One thing to note is the capitalisation of the button text is now different. But this makes sense with the code changes and the styles package's detachment but obviously doesn't follow Material Design

[oliviertassinari]

@joshwooding Correct. @material-ui/styles support no specific Design Specification. Here is Gmail 😆