fix: use nav elements with descriptive aria-labels

[jendowns]

Description

This PR sets out to make 2 related improvements to the navigation of the site. Please let me know what you think -- thank you!

1. Ensure that all navigation components use a <nav> element

I noticed that docs page has a secondary navigation wrapped in an <aside>.

In this case I believe it would be more helpful to use <nav> to more directly signal the purpose of the markup, as it includes navigation links (reference: <nav>: The Navigation Section element via MDN). By comparison, an <aside> element is associated with complementary content "indirectly related to the document's main content" (reference: <aside>: The Aside element via MDN).

2. Ensure that all <nav> elements have descriptive aria-label attributes

There are instances where multiple <nav>s appear on a page, as is the case with the newly-revised Docs page and the current News page.

Whenever there are multiple <nav>, it helps to either point to an existing label (via aria-labelledby) or create a label (via aria-label). For this PR, I opted to use aria-label because I did not see any existing headings that would be ideal to point to with aria-labelledby. Related resource: WCAG 2.1 -- Example using aria-label to distinguish between navigation landmarks

[DerekNonGeneric]

This looks great — thanks so much for doing this!

/cc @Trott