API docs styling tweaks

[lukasnys]

Fixes #879 as well as some other styling tweaks:

• Use sidebar-container--full-width (Start using "wide mode" in the redesign branch #879) and add necessary padding
• Match sidebar font-size with guidemaker-ember-template (PR: Sidebar styling tweaks guidemaker-ember-template#162)
• Make header full-width
• Increase sidebar width so items don't overflow

Let me know if one of the additional styling tweaks (font-size changes, header width changes, sidebar width changes) are undesired as I didn't talk this through with anyone.

Screenshots before and after

x	Before	After
Desktop
Mobile
Mobile sidebar

[lukasnys]

One thing I'm concerned with in this PR is the --sidebar-width: min-content. I added it to stop sidebar items from overflowing their parent, however if the sidebar items ever become smaller in size, it might cause the layout to "break"

[MinThaMie]

Am I correct when I understand that you made it min-content because the sidebar-items were not wrapping? Because I think we would love it if the sidebar-width is consistent between all the "apps"/websites and using min-content does not guarantee that. So if that's what you are aiming for, I suggest using word-wrap.