Update 'Import JavaScript' page to use new `createAll` function

[romaricpascal]

[!WARNING] This PR should only be merged after the next release of GOV.UK Frontend

Updates the Frontend docs to reflect the addition of a new createAll function to initialise all instances of a specific component on the page.

Also moves the 'Initialise only part of a page' to the 'Import JavaScript' page, rather than 'Configure components' to regroup both how to import and initialise components.

Each commit details the changes of a specific page.

Closes alphagov/govuk-frontend#4945

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