[MRG] Have azure ondemand

[TimotheeMathieu]

The goal of this PR is to be able to trigger the azure pipeline by labelling the PR as "ready for review". This is more eco-friendly than running the checks at each commit.

For now, read the docs checks are still automatically done at each commit. I think it is not possible to check the label automatically. The eco-friendly solution would be either to deactivate it or to try to make it faster by having the examples be faster or by pre-running the examples and using images when the doc is built.

EDIT: it seems to works. The labelling does not trigger the checks, instead the checks are run only if the label is set (so you have to either set the label before the commit you want to check or you have to recommit something to trigger CI).