Update navbar and ToC to work well with the fixed content width

In !942 (merged), we shipped the first iteration of the content width of the docs site. We still need to handle:

1. Bring in left sidebar as per the design reference: https://www.figma.com/file/e3QXi7cwhN2TyzoG8IWPMD/Docs-North-%E2%AD%90%EF%B8%8F?node-id=0%3A1
2. Update the right sidebar designs and HTMl structure
3. Mobile => If we are hiding the left side bar on internal pages how do we navigate the site on mobile?

Based on the figma reference (see first bullet), the ending result will be: