Fix style guide links in CONTRIBUTING.md
Fix style guide links in CONTRIBUTING.md

We opted to just drop the CSS style guide for now because we don’t
really follow one. @simurai please correct that if I’m wrong.

Closes #12066

Signed-off-by: Max Brunsfeld <78036c9b69b887700d5846f26a788d53b201ffbb@github.com>

kevinrenaers/atom,andrewleverette/atom,rlugojr/atom,sotayamashita/atom,stinsonga/atom,liuderchi/atom,atom/atom,decaffeinate-examples/atom,andrewleverette/atom,me-benni/atom,CraZySacX/atom,FIT-CSE2410-A-Bombs/atom,xream/atom,Mokolea/atom,decaffeinate-examples/atom,FIT-CSE2410-A-Bombs/atom,Ingramz/atom,AdrianVovk/substance-ide,brettle/atom,bsmr-x-script/atom,tjkr/atom,t9md/atom,atom/atom,stinsonga/atom,bolinfest/atom,PKRoma/atom,kevinrenaers/atom,brettle/atom,AlexxNica/atom,ardeshirj/atom,Mokolea/atom,Arcanemagus/atom,bolinfest/atom,brettle/atom,Arcanemagus/atom,PKRoma/atom,liuderchi/atom,AlexxNica/atom,PKRoma/atom,andrewleverette/atom,ardeshirj/atom,AdrianVovk/substance-ide,liuderchi/atom,xream/atom,Ingramz/atom,Arcanemagus/atom,rlugojr/atom,me-benni/atom,AdrianVovk/substance-ide,AlexxNica/atom,decaffeinate-examples/atom,tjkr/atom,rlugojr/atom,atom/atom,sotayamashita/atom,Ingramz/atom,bolinfest/atom,FIT-CSE2410-A-Bombs/atom,Mokolea/atom,sotayamashita/atom,decaffeinate-examples/atom,t9md/atom,xream/atom,CraZySacX/atom,me-benni/atom,tjkr/atom,ardeshirj/atom,bsmr-x-script/atom,stinsonga/atom,kevinrenaers/atom,bsmr-x-script/atom,liuderchi/atom,t9md/atom,CraZySacX/atom,stinsonga/atom