[Dev] release v0.3.1

[jbcaillau]

@ocots what to do to have the new release seen as stable in the doc?

[jbcaillau]

@ocots it seems to be updated now: is it automatic once there is a new tag?

PS. the badge for release / version does not show properly in https://github.com/control-toolbox/Tutorials.jl

[ocots]

When you make a new release in the GitHub sense, the stable doc is automatically updated. Just be sure to be consistent with Project.toml.

[ocots]

For the badge, there is a problem since it is not a Julia package. I have changed all the release badges to get an image from GitHub releases instead of Julia.