Remove "responsive" Header examples unless they are usable on all viewport sizes

[montchr]

Only one two of the Header examples actually work on small viewports, and yet there are "twelve" "responsive" components. This feels very misleading and confusing. What's more is that it's entirely unclear which of these components has a menu toggle that works at smaller viewport widths.

At first I thought every menu in the Header components was broken and came here to look for bug reports, but after finding #37 I see that this is apparently expected behavior. As a prospective user of the library, it makes me question the integrity of the other components in Mantine: i.e. am I going to be unpleasantly surprised to find that something else doesn't work as it would appear on the surface?

Regardless of whether the headers are labelled "responsive" or not: what is the point of collapsing a menu behind a toggle button on mobile if the toggle button does nothing at all? I would suggest removing the non-functional components entirely until they can be re-introduced with the functionality implied by the menu toggle button. Otherwise, the examples aren't really showing anything interesting at all – it's easy to toggle the state of an icon, but mobile navigation is hard.