DOC: Add style guide to howto_document
DOC: Add style guide to howto_document

Fixes gh-16262

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