Clear out directory after docs build and remove stale previews

[efaulhaber]

When cloning TrixiParticles.jl, we already have to download 200MB, the majority of which is the gh-pages branch. Most of that comes from the VTK files that are generated during the doctests and literate tutorials, which are all committed to gh-pages in every single docs build, including PR previews.

This PR

• adds a line to automatically clear the VTK output directory of literate simulations before uploading to the docs.
• adds a GitHub action to automatically remove stale docs previews (standard code copied from https://documenter.juliadocs.org/stable/man/hosting/#Cleaning-up-gh-pages).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.49%. Comparing base (26c5fcc) to head (48f95c4).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1300   +/-   ##
=======================================
  Coverage   90.49%   90.49%           
=======================================
  Files         139      139           
  Lines       11337    11337           
=======================================
  Hits        10259    10259           
  Misses       1078     1078           

Flag	Coverage Δ
total	90.49% <ø> (ø)
unit	77.58% <ø> (ø)

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[efaulhaber]

Codex Review

The PR removes generated tutorial outputs before documentation deployment and adds closed-PR preview cleanup with history compaction.

This is an AI-generated code review. Please verify the findings and summary before acting on them.
_{Review generated by codex-pr-review}