Add custom 404 page and verify essential site files in CI #4295

Job	Run time
Detect File Changes	11s
Lint Gate	1m 23s
Github (ubuntu, mpi, debug, true)	36s
Github (ubuntu, mpi, debug, false)	28s
Georgia Tech \| Phoenix (gpu-omp)	25s
Github (ubuntu, mpi, no-debug, true)	37s
Oak Ridge \| Frontier (AMD) (gpu-omp)	25s
Oak Ridge \| Frontier (gpu-acc)	1s
Georgia Tech \| Phoenix (cpu)	1s
Georgia Tech \| Phoenix (gpu-acc)	25s
Oak Ridge \| Frontier (AMD) (cpu)	1s
Github (ubuntu, mpi, no-debug, false)	30s
Oak Ridge \| Frontier (cpu)	1s
Github (macos, mpi, debug, false)	1s
Github (macos, mpi, no-debug, false)	25s
Github (ubuntu, no-mpi, single, no-debug, false)	27s
Oak Ridge \| Frontier (gpu-omp)	1s
	5m 58s

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