[docs] Move the themes to themes.material-ui.com

[oliviertassinari]

For now, http://themes.material-ui.com is a simple replication of https://material-ui.com/premium-themes/. However, it comes with a multi-vendor e-commerce backend behind the scene we will be able to leverage going forward (daed976).

[mui-pr-bot]

Details of bundle changes.

Comparing: d533b46...b66b78b

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	317,442	317,442	86,957	86,957
@material-ui/core/Paper	0.00%	0.00%	67,922	67,922	20,181	20,181
@material-ui/core/Paper.esm	0.00%	0.00%	61,217	61,217	18,977	18,977
@material-ui/core/Popper	0.00%	0.00%	28,728	28,728	10,345	10,345
@material-ui/core/Textarea	0.00%	0.00%	5,513	5,513	2,378	2,378
@material-ui/core/TrapFocus	0.00%	0.00%	3,755	3,755	1,575	1,575
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	15,978	15,978	5,786	5,786
@material-ui/core/useMediaQuery	0.00%	0.00%	2,106	2,106	974	974
@material-ui/lab	0.00%	0.00%	138,855	138,855	42,673	42,673
@material-ui/styles	0.00%	0.00%	51,386	51,386	15,193	15,193
@material-ui/system	0.00%	0.00%	14,463	14,463	4,178	4,178
Button	0.00%	0.00%	83,901	83,901	25,467	25,467
Modal	0.00%	0.00%	20,342	20,342	6,688	6,688
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	+0.08% 🔺	+0.14% 🔺	55,364	55,410	13,911	13,931
docs.main	0.00%	0.00%	649,683	649,683	205,139	205,139
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	291,336	291,336	83,266	83,266

Generated by 🚫 dangerJS against b66b78b