[Dialog]: Change Storybook example headings to h1

[RiinaKuu]

Jira-ticket: https://funidata.atlassian.net/browse/DS-321

• Changed Storybook and docs Dialog examples to start from h1

There are discussions whether dialogs should start its semantic heading levels from 1 or 2 (or to match it with where it originally opened). However, in the IAAP WAS certificate training it was stated as follows:

There ought to be only one heading level 1, if possible, though this is not an absolute rule. A dialog should also start with a heading level 1, because a dialog is essentially a stand-alone view, almost like a mini web page.