Proj_jll grid files

[evetion]

Based on the discussion in https://github.com/JuliaGeo/Proj.jl/issues/75 and my troubles to get the CI working for PROJ_NETWORK in https://github.com/evetion/GeoArrays.jl/pull/126, we should try to document how downstream packages can make sure the PROJ grid files are available (remotely).

For example, GeoArrays does not depend on Proj or Proj_jll directly, only indirectly on the jll based on GDAL (via ArchGDAL). Currently I try to just PROJ_NETWORK=ON and hope for the best. Ideally, this works out of the box, like Proj_jll running projsync on init or automatically execute things like proj_context_set_search_paths into the package.