DemoCards crashes on cleanup on Windows

[tlnagy]

First of all thanks @johnnychen94 for the awesome DemoCards package, but I'm running into a bug specifically on Windows where DemoCards crashes in the cleanup callback

From the Windows CI logs for https://github.com/tlnagy/Nagy_2023_SwellMigration :

[ Info: Clean up DemoCards build dir: "..\figures"
ERROR: LoadError: IOError: rm("D:\\a\\Nagy_2023_SwellMigration\\Nagy_2023_SwellMigration\\site\\src\\figures\\Notebooks"): resource busy or locked (EBUSY)
Stacktrace:
 [1] uv_error
   @ Base .\libuv.jl:100 [inlined]
 [2] rm(path::String; force::Bool, recursive::Bool)
   @ Base.Filesystem .\file.jl:307
 [3] rm(path::String; force::Bool, recursive::Bool)
   @ Base.Filesystem .\file.jl:294
 [4] rm
   @ .\file.jl:273 [inlined]
 [5] (::DemoCards.var"#113#118"{String, String, String, String})()
   @ DemoCards C:\Users\runneradmin\.julia\packages\DemoCards\Oz6IE\src\generate.jl:209
 [6] top-level scope
   @ D:\a\Nagy_2023_SwellMigration\Nagy_2023_SwellMigration\site\make.jl:47
in expression starting at D:\a\Nagy_2023_SwellMigration\Nagy_2023_SwellMigration\site\make.jl:47
GKS: could not find font bold.ttf
Error: Process completed with exit code 1.

This appears to be triggered by these lines: https://github.com/JuliaDocs/DemoCards.jl/blob/6dde0a3c912b17c2db173158882fbb35c62fd478/src/generate.jl#L208-L209

Perhaps the GC.gc() command isn't freeing up the resource any more?