Fix Pages deployment: preserve PDF in HTML output

[ggorman]

Summary

• Fix publish workflow to preserve PDF when building HTML output
• The previous workflow built HTML first, then PDF, which meant only PDFs were in the final artifact
• Now builds PDF first, saves it, builds HTML, then copies PDF back

Test plan

• Merge and verify https://devitocodes.github.io/devito_book/ shows HTML content
• Verify PDF download link works

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@24bc811). Learn more about missing BASE report.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage        ?   48.18%           
=======================================
  Files           ?        4           
  Lines           ?      330           
  Branches        ?       48           
=======================================
  Hits            ?      159           
  Misses          ?      158           
  Partials        ?       13           

Flag	Coverage Δ
derivations	48.18% <ø> (?)

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