DOC: update roadmap
DOC: update roadmap

Removes completed items and other outdated content, plus some things
that are too detailed or not high-prio enough to be on the roadmap.
Add new items that are being actively worked on or for which there's a
lot of interest. Link the most relevant NEPs.

madphysicist/numpy,pdebuyl/numpy,seberg/numpy,seberg/numpy,pbrod/numpy,mattip/numpy,pbrod/numpy,jakirkham/numpy,endolith/numpy,seberg/numpy,madphysicist/numpy,mhvk/numpy,simongibbons/numpy,endolith/numpy,anntzer/numpy,charris/numpy,anntzer/numpy,numpy/numpy,seberg/numpy,grlee77/numpy,endolith/numpy,pbrod/numpy,jakirkham/numpy,anntzer/numpy,mattip/numpy,rgommers/numpy,pdebuyl/numpy,pdebuyl/numpy,mattip/numpy,pbrod/numpy,mhvk/numpy,jakirkham/numpy,rgommers/numpy,mattip/numpy,charris/numpy,charris/numpy,jakirkham/numpy,grlee77/numpy,numpy/numpy,charris/numpy,mhvk/numpy,pdebuyl/numpy,simongibbons/numpy,numpy/numpy,mhvk/numpy,grlee77/numpy,pbrod/numpy,anntzer/numpy,simongibbons/numpy,simongibbons/numpy,endolith/numpy,simongibbons/numpy,madphysicist/numpy,jakirkham/numpy,rgommers/numpy,grlee77/numpy,rgommers/numpy,mhvk/numpy,madphysicist/numpy,madphysicist/numpy,grlee77/numpy,numpy/numpy