Mention availability of a built CSS file in the package

[romaricpascal]

What

Add a mention for the govuk-frontend.min.css file in our guidance:

• On the Importing CSS assets and JavaScript page
• Possibly on the Get Started page as well, as copying a file (like we do for the JavaScript) is a simpler way to get started. Will have to be balanced with our preference for people only importing the parts they need (though that bit is buried in the Importing CSS assets and JavaScript page)

Why

alphagov/govuk-frontend#4240 adds a built CSS file to our package. It would be helpful for it to be listed in the frontend docs so people know to use it (if they don't have Sass on their project).

Who needs to know about this

Done when

• Update the Importing CSS assets and JavaScript page
• Decide whether to update the Get Started page as well, as copying a file (like we do for the JavaScript) page and update content if necessary