[redesign] Improve left-nav scan-ability

[guidezpl]

Feedback for https://github.com/flutter/website/pull/11172

The amount of content in the left nav is extensive (good) and I think we could improve the ability of users to find what they want for the nav bar in a few ways:

• differentiate category headers (e.g. User interface) more; perhaps with some indent/unindent or larger, bolder type
• add icons to categories (e.g. Design and theming). Given the level of specificity for some categories, may require a specialized or custom icon set

Also

• the Reference category is expanded by default, not sure why. I think this could easily be nested under Resources
• the Resources category subheaders (e.g. Learning) aren't differentiated enough from items