Make scrollbar placement more natural when using `sidebar`

[MangelMaxime]

Hello, This is a proposition.

When using sidebar, a margin-top is automatically applied to the "main content" of the page.

This cause the scrollbar to be placed in a not natural position:

I think that this extra margin-top should be removed so the scrollbar is placed naturally at the top of the page.

https://github.com/bigskysoftware/missing/blob/97451eaafcd147a5e9d74ac6ff30969bc299c7fd/src/components.css#L91-L95

In general, there will naturally be a spacing at the top of the page because it should in general start with a heading element or a main to center the content on the page. And if not, user can add the spacing if needed.