Add style standard to style doc

[cezaraugusto]

Auditors: @luixxiul
Close #8340

We have standardjs to take care of coding practices but we're still loose about how we deal with styles. As codebase grows it gets harder to follow a convention. This update adds new coding practices while writing styles and enforce using our adapted BEM pattern.

[NejcZdovc]

@cezaraugusto can we put this into linter? eslint custom rule maybe?

[cezaraugusto]

I was thinking about that yesterday, a custom eslint rule would be ideal. We can take StyleSheet.create as the identifier and work from there looking for spaces/quotes and other rules we want to check.

Didn't have time to look closer but worth considering, so ++. I created an issue to track this #9181

[luixxiul]

I would add the reason why we are going to choose BEM over other methodologies (1. following BEM should avoid visual regressions nicely, 2. BEM makes it possible for us to make preparations for style nesting in advance, though it is not sure if it could be implemented to Aphrodite).

Also I found a typo on the file. I'm going to fix it and push the fix.

[luixxiul]

This is the fix: https://github.com/luixxiul/browser-laptop/commit/0bfcb6ad7e1156b2e71b35bc2cdd4bbffcdcff93

@cezaraugusto please cherry-pick and squash it, thanks.

[luixxiul]

Feedback left.

[cezaraugusto]

thanks, added some reasons on why we choose BEM, feel free to edit if you have more/better ones

[luixxiul]

the instruction looks nice!