Welcome Guide: modal should be labelled

[enriquesanchez]

Describe the bug
From #18041 (comment)

The modal is not labelled. This is in part a problem with this modal, but should probably be corrected in the component: the Modal component allows the component to be misused.

We should enforce a label for the modal element with role=dialog within the e2e tests, because all modals must be labelled. Additionally, the modal component should enforce role="document" on the content of all modal dialogs. This isn't strictly required on all modals, but the component can't know what type of content a modal will contain, this will give the best overall experience.