refactor(docs-infra): scope select-menu styles to the `<aio-select>` element (#40704)
refactor(docs-infra): scope select-menu styles to the `<aio-select>` element (#40704)

This commit ensures that all styles for the select-menu element (which
are defined in `_select-menu.scss`) only apply to elements inside an
`<aio-select>` element. This will prevent the styles accidentally taking
effect on a different part of the app.

PR Close #40704

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