feat: add a skip nav link to improve keyboard navigation

[chrisvogt]

This PR adds a skip nav link using @reach/skip-nav to improve keyboard navigation. As you navigate between pages, it's cumbersome to TAB through the header links every page. After this change, a new button appears at the top of the page if when you first begin to TAB through the content, allowing you to jump to the main content area.

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[chrisvogt]

I'm holding off on merging this because it's not consistently working as expected: a large number of times it doesn't skip focus and just scrolls to the main content element.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 76.46%. Comparing base (2a2a233) to head (a64a53c).
Report is 15 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #212   +/-   ##
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  Coverage   76.46%   76.46%           
=======================================
  Files          66       66           
  Lines         667      667           
  Branches      167      167           
=======================================
  Hits          510      510           
  Misses        143      143           
  Partials       14       14           

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