fix(docs-infra): fix width of sidenav icons (#42561)
fix(docs-infra): fix width of sidenav icons (#42561)

make sure that the width of the sidenav chervon icon is 2.4rem
(this needs to be done using the flex property and not the
width one as that can change in flex containers)

also center chevron icon inside mat-icon container in order to maintain
the correct icon positioning at any font-size

PR Close #42561

gnomeontherun/angular,mgechev/angular,mgechev/angular,ocombe/angular,angular/angular,ocombe/angular,angular/angular,brandonroberts/angular,mgechev/angular,gkalpak/angular,gkalpak/angular,ocombe/angular,angular/angular,angular-indonesia/angular,shairez/angular,brandonroberts/angular,angular/angular,mgechev/angular,gnomeontherun/angular,JiaLiPassion/angular,manekinekko/angular,shairez/angular,JiaLiPassion/angular,ocombe/angular,brandonroberts/angular,angular-indonesia/angular,brandonroberts/angular,brandonroberts/angular,gnomeontherun/angular,ocombe/angular,gkalpak/angular,mgechev/angular,manekinekko/angular,gkalpak/angular,gnomeontherun/angular,brandonroberts/angular,manekinekko/angular,manekinekko/angular,shairez/angular,angular-indonesia/angular,ocombe/angular,manekinekko/angular,shairez/angular,JiaLiPassion/angular,angular-indonesia/angular,manekinekko/angular,JiaLiPassion/angular,shairez/angular