infra: add permissions to _site for jekyll

[sesmart]

Issue #, if available:

Fixes permission error for docker container; see actions/jekyll-build-pages#101.

Description of changes:

Testing done:

Merge Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ea8e916) to head (6c64830).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1303   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          170       170           
  Lines        11011     11011           
  Branches      1413      1413           
=========================================
  Hits         11011     11011           

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