Squashes the changes from #17921 and makes some additional changes:

• Updates the docs to include disabled example
• Clarifies some component behavior in the docs
• Rewrites the styles to use less nesting and more specific class names
• Overhaul the custom file input with nesting and more specific classes
• Change custom select menu's class to .custom-select
• Fix the rounding on the custom select menus

/cc @markau