Clarify that copied JavaScript and CSS files need to be served by the application

[romaricpascal]

What

Clarify the steps of our Get Started page asking to copy GOV.UK Frontend's CSS (step 1) and JavaScript (step 2) files into the project to explain that the folder you copy them to needs to be served by your application.

Why

While we can't technically explain how to serve the files, users have had issues running getting started because the copied files were not served by their server.

Who needs to know about this

Developers, Tech writers

Done when

• We've updated the Get Started page
• We've looked for other occurences of asking to copy GOV.UK Frontend page in the docs that could use a similar clarification