Update documentation to reference new ways of importing components

[36degrees]

What

• Update the guidance on importing CSS, assets and JavaScript to reflect the changes in Allow components to be imported with or without dependencies govuk-frontend#1797.

• Check if there are other parts of the documentation that also need to be updated.

Further detail

With the changes in alphagov/govuk-frontend#1797, the preferred way to import a subset of the components will be to do something like:

@import "govuk/base";

@import "govuk/components/button/index"; 
@import "govuk/components/details/index"; 

If using a Sass compiler that supports index files, this could just be:

@import "govuk/base";

@import "govuk/components/button"; 
@import "govuk/components/details"; 

You can still import a single component without having to separately import its dependencies:

@import "govuk/components/button/button";

However, this is only really meant to be used when you need to import a single component in isolation, e.g. if you're creating a React component.

Why

Importing the components using the index file avoids re-importing the 'base' (settings, tools and helpers layers) multiple times, improving compilation times.

Who needs to know about this

@m-green, @36degrees, @hannalaakso