[core] Batch small changes

[oliviertassinari]

• [docs] Only bootstrap in the _app.js: I believe the logic was introduced in v3 for: https://v3.material-ui.com/css-in-js/basics/#migration-for-material-ui-core-users.
• [test] Apply defaultProps convention: quick search & replace, low low value
• [docs] Button explicit react-router: There is no result for the query react-router in Algolia.
• [docs] Explain why you might want to use JSS: Related to makeStyles: Cannot successfully combine props with selectors #16745 (comment).
• [docs] Replace properties -> props: I should have converted all the occurrences that we don't translate. I don't plan on changing the translated ones (in the markdown). I won't have the courage for it (i18n keys invalidation will be an issue).
• [docs] Fix GA events: Improve the details of the current tagging.
• [docs] Sort demos by popularity: Used GA events for the new documentation pages

[mui-pr-bot]

Details of bundle changes.

Comparing: e26e3b4...daabcfe

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	331,368	331,368	90,896	90,896
@material-ui/core/Paper	0.00%	0.00%	69,177	69,177	20,528	20,528
@material-ui/core/Paper.esm	0.00%	0.00%	62,047	62,047	19,208	19,208
@material-ui/core/Popper	0.00%	0.00%	29,182	29,182	10,427	10,427
@material-ui/core/Textarea	0.00%	0.00%	5,759	5,759	2,367	2,367
@material-ui/core/TrapFocus	0.00%	0.00%	3,806	3,806	1,602	1,602
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,390	16,390	5,825	5,825
@material-ui/core/useMediaQuery	0.00%	0.00%	3,221	3,221	1,314	1,314
@material-ui/lab	0.00%	0.00%	141,987	141,987	43,790	43,790
@material-ui/styles	0.00%	0.00%	51,886	51,886	15,339	15,339
@material-ui/system	0.00%	0.00%	15,761	15,761	4,380	4,380
Button	0.00%	0.00%	79,460	79,460	24,301	24,301
Modal	0.00%	0.00%	14,961	14,961	5,221	5,221
Portal	0.00%	0.00%	3,579	3,579	1,566	1,566
Rating	0.00%	0.00%	70,667	70,667	22,064	22,064
Slider	0.00%	0.00%	75,061	75,061	23,277	23,277
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	-0.02%	-0.10%	52,056	52,044	13,831	13,817
docs.main	-2.67%	-2.90%	606,828	590,609	194,402	188,769
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	299,987	299,987	86,202	86,202

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