Align Skip link

[chris-gds]

What?

As part of this PR discussion it was noticed that noticed that the "Skip Link" anchor link is not as consistent as it could be across frontend (and perhaps other repos)

GOV.UK use:

https://github.com/alphagov/govuk_publishing_components/blob/3b15e3c69698e2f18d95e9e59e2f998dad537779/app/views/govuk_publishing_components/components/_layout_for_public.html.erb#L66 uses #content

https://components.publishing.service.gov.uk/component-guide/skip_link uses #main-content

Design System use

https://design-system.service.gov.uk/styles/page-template/ uses #main-content

https://design-system.service.gov.uk/components/skip-link/ uses #content