docs: Include Posit badge in header of project website

[rich-iannone]

This adds the 'Supported by Posit' badge in the header of the project website (https://posit-dev.github.io/supported-by-posit/).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.45%. Comparing base (140adee) to head (1a6278a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #777   +/-   ##
=======================================
  Coverage   91.45%   91.45%           
=======================================
  Files          47       47           
  Lines        5558     5558           
=======================================
  Hits         5083     5083           
  Misses        475      475           

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