use julia-actions/cache also for Documenter

[ranocha]

See trixi-framework/Trixi.jl#1802. This should speed up building the docs.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (85b75ee) 98.85% compared to head (e14e375) 98.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          20       20           
  Lines        1838     1838           
=======================================
  Hits         1817     1817           
  Misses         21       21           

Flag	Coverage Δ
unittests	98.85% <ø> (ø)

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[coveralls]

coverage: 98.857%. remained the same
when pulling e14e375 on ranocha-patch-1
into 85b75ee on main.