Improves RMG-Py cloning in CI & fixes website build

[calvinp0]

Addresses an issue where the RMG-Py repository was being cloned repeatedly during CI runs.

• Optimizes the CI workflow by checking for the existence of the RMG-Py directory before attempting to clone it.
• If the directory exists, the workflow now fetches and checks out the specified commit instead of re-cloning the entire repository.

Website deployment is now pointed to the ./ARC/ folder

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.49%. Comparing base (9ba9803) to head (2dbd810).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #796   +/-   ##
=======================================
  Coverage   74.49%   74.49%           
=======================================
  Files         103      103           
  Lines       28121    28121           
  Branches     5771     5771           
=======================================
  Hits        20950    20950           
  Misses       5680     5680           
  Partials     1491     1491           

Flag	Coverage Δ
unittests	74.49% <ø> (ø)

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