Filter doctests instead of preloading packages

[andreasnoack]

If a sparse matrix isn't in the local MatrixDepot cache then the matrixdepot function outputs some extra lines related to the download that causes doctests failure. To work around that, I've added a line that downloads the matrix as a part of the docs setup, see https://github.com/JuliaLinearAlgebra/TSVD.jl/blob/2d8f0d4cdc385cd93c5566ab72b8373dad2bbff1/docs/make.jl#L6 and https://github.com/JuliaLinearAlgebra/TSVD.jl/blob/2d8f0d4cdc385cd93c5566ab72b8373dad2bbff1/.github/workflows/CI.yml#L67. It would be better to use the filtering functionality in Documenter to filter this out. However, it requires that I figure out how to write the regex.