Background
Currently the elements that sit inside the sidebar (main menu and ToC) are have styles that apply widths, which also depend on the viewport (located in Sidebar.less). Rather than apply these styles independently, we should apply the width to the grid area that contains the sidebar. This simplifies the CSS and allows the TOC to take up the full space regardless if it has a negative margin
AC
- main menu and TOC no longer have width styles
- The 'sidebar' grid template area has the correct width styles
