DOC: document how to skip CI jobs
DOC: document how to skip CI jobs

This is a follow-up to gh-18333.

Self-test: docs get built on CircleCI, so let's skip the other
platforms:

[skip github]
[skip azurepipelines]
[skip appveyor]
[skip travis]

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