Add Arch Linux User Repository (AUR) installation instructions to documentation #8389

Job	Run time
filter-changes	5s
coverage	4s
Python ${{ matrix.python-version }} (omp=${{ matrix.omp }}, mpi=${{ matrix.mpi }}, dagmc=${{ matrix.dagmc }}, libmesh=${{ matrix.libmesh }}, event=${{ matrix.event }}	0s
Check CI status	4s
	13s

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