docs: document how external link icons work (#37025)
docs: document how external link icons work (#37025)

After PR #36601 which added icons to all external links. Documented how this is happening via comments in scss file. For details visit PR #36601

PR Close #37025

IdeaBlade/angular,wKoza/angular,ocombe/angular,brandonroberts/angular,gnomeontherun/angular,ocombe/angular,manekinekko/angular,gkalpak/angular,filipesilva/angular,shairez/angular,blesh/angular,ocombe/angular,ocombe/angular,angular/angular,angular/angular,IdeaBlade/angular,filipesilva/angular,angular-indonesia/angular,wKoza/angular,shairez/angular,manekinekko/angular,gkalpak/angular,ocombe/angular,alxhub/angular,JiaLiPassion/angular,brandonroberts/angular,alxhub/angular,wKoza/angular,gdi2290/angular,mgechev/angular,blesh/angular,IdeaBlade/angular,manekinekko/angular,JiaLiPassion/angular,shairez/angular,angular/angular,gkalpak/angular,brandonroberts/angular,IdeaBlade/angular,gnomeontherun/angular,angular-indonesia/angular,shairez/angular,blesh/angular,angular-indonesia/angular,filipesilva/angular,wKoza/angular,mgechev/angular,IdeaBlade/angular,alxhub/angular,alxhub/angular,mgechev/angular,JiaLiPassion/angular,alxhub/angular,brandonroberts/angular,brandonroberts/angular,ocombe/angular,angular-indonesia/angular,blesh/angular,manekinekko/angular,gdi2290/angular,gdi2290/angular,filipesilva/angular,manekinekko/angular,angular/angular,gnomeontherun/angular,gdi2290/angular,mgechev/angular,shairez/angular,gkalpak/angular,brandonroberts/angular,JiaLiPassion/angular,manekinekko/angular,gdi2290/angular,gnomeontherun/angular,blesh/angular,wKoza/angular,gdi2290/angular,mgechev/angular,wKoza/angular,filipesilva/angular