fix(layout): increase gap between sidebar and content

[LeoMcA]

A fix for #690 (comment), increasing the gap between content and the sidebar content by ~1rem to 3rem

Also fixes #593: left sidebar no longer overlaps with header anchor hashes

[github-actions]

0ae1ccd was deployed to: https://fred-pr1154.review.mdn.allizom.net/

[LeoMcA]

@Rumyra this PR increases the gap between sidebar and content by 1rem:

before:

vs after:

I'd recommend viewing the images fullscreen, or following the links to the pages themselves. It's easy to increase the gap by even more if you want: modify --layout-sidebar-gap in devtools and see what looks good.