DOC: add instructions for cross compilation
DOC: add instructions for cross compilation

Closes #20737.

mattip/numpy,rgommers/numpy,numpy/numpy,numpy/numpy,pdebuyl/numpy,charris/numpy,seberg/numpy,jakirkham/numpy,charris/numpy,numpy/numpy,seberg/numpy,mhvk/numpy,anntzer/numpy,anntzer/numpy,pdebuyl/numpy,pdebuyl/numpy,seberg/numpy,rgommers/numpy,endolith/numpy,jakirkham/numpy,jakirkham/numpy,rgommers/numpy,jakirkham/numpy,mattip/numpy,endolith/numpy,mhvk/numpy,mhvk/numpy,seberg/numpy,charris/numpy,anntzer/numpy,mhvk/numpy,mattip/numpy,jakirkham/numpy,endolith/numpy,numpy/numpy,endolith/numpy,mhvk/numpy,pdebuyl/numpy,charris/numpy,anntzer/numpy,mattip/numpy,rgommers/numpy