Add logging docs, `FileLogger`, `OnlyRootLogger`

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:pipeline:buildkite-agent pipeline upload
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initializeecho "--- Instantiate project" && julia --project=test -e 'using Pkg; Pkg.develop(;path="."); Pkg.add("CUDA"); Pkg.add("MPI"); Pkg.instantiate(;verbose=true); Pkg.precompile(;strict=true)' && julia --project=test -e 'using CUDA; CUDA.precompile_runtime()' && julia --project=test -e 'using Pkg; Pkg.status()'
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馃捇 testsjulia --project=test test/runtests.jl
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馃捇 tests MPIsrun julia --project=test test/runtests.jl
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馃捇 threaded testsjulia --threads 4 --project=test test/runtests.jl
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馃捇 threaded tests MPIsrun julia --threads 4 --project=test test/runtests.jl
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馃幋 testsjulia --project=test test/runtests.jl
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馃幋 tests MPIsrun julia --project=test test/runtests.jl
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Total Job Run Time: 5m 20s